Adultery is the voluntary or consensual sexual intercourse between a married person (the husband or wife) and a person (a third party whether married or unmarried) of the opposite sex, not being the other’s spouse. It should be noted that homosexual intercourse or other forms of sexual activity not involving sexual intercourse do not constitute adultery; Dennis v Dennis  P 153.
Some territories require that divorce petitions relying on the fact of adultery make the alleged adulterer the co-respondent, while some territories at the discretion of the court can make the adulterer pay whole or part of the proceedings.
The standard of proof required is on a balance of probabilities, but because adultery was seen as a serious matrimonial offence akin to criminal offences, some courts required proof beyond a reasonable doubt or a very high standard of proof to satisfy the court that the offence was committed.
In the case of Bastable v Bastable and Sanders  1 WLR 1684 The parties were married in 1949 and were social friends with the co-respondent and his wife from 1963. While the husband and the co-respondent’s wife were both working away from home a good deal during the day, and the wife and the co-respondent were not working they associated with each other freely as friends. The couple separated in 1965 and in proceedings for divorce on the ground of desertion by the wife (which was not contested), the husband also accused her of adultery with the co-respondent.
There was no evidence of any affectionate behaviour between the wife and the correspondent or that they had ever been caught in a compromising position. The Court of Appeal held that although adultery need not be proved beyond all reasonable doubt, it was an offence (albeit a matrimonial one). The court bore in mind the high standard of proof required in a divorce suit on the ground of adultery on the principle that “in proportion as the offence is grave so ought the proof to be clear”. The appeal was allowed for in the instant case since the husband had not adduced evidence sufficient to discharge the onus on him and had not proved adultery. Suspicion is not enough.
Wilmer LJ-applying the test formulated by Denning LJ stated in Bater v Bater  P 35, 37 the following:
““The difference of opinion which has been evoked about the standard of proof in recent cases may well turn out to be more a matter of words than anything else. It is, of course true that by our law a higher standard of proof is required in criminal cases than in civil cases. But this is subject to the qualification that there is no absolute standard in either case. In criminal cases the charge must be proved beyond reasonable doubt, but there may be degrees of proof within that standard. As many great judges have said: ‘in proportion as the crime is enormous, so ought the proof to be clear.’ So also in civil cases, the case may be proved by a preponderance of probability, but there may be degrees of probability within that standard. The degree depends on the subject-matter. A civil court, when considering a charge of fraud, will naturally require for itself a higher degree of probability than that which it would require when asking if negligence is established. It does not adopt so high a degree as a criminal court, even when it is considering a charge of a criminal nature; but still it does require a degree of probability which is commensurate with the occasion. Likewise, a divorce court should require a degree of probability which is proportionate to the subject-matter.”
The Matrimonial Proceedings and Property Act Chapter 45:51 of Trinidad and Tobago shows that the court hearing a petition of divorce shall not hold the marriage to have broken down irretrievably unless the petitioner satisfies the court of one or more of the listed facts in Section 4 of the Act. The Act however makes no distinction between the standard of proof required for adultery and that of any of the facts constituting irretrievable breakdown and the standard of proof may be proved by the preponderance of probability. To succeed, however, the petitioner does not necessarily have to prove the direct fact or even an act of adultery in time and place. The fact can be inferred from circumstances, which lead to it by fair inference, as a necessary conclusion. The court has to be satisfied that there was something more than opportunity before it will find that adultery has been committed.
Ultimately, in the Trinidad and Tobago case of Leong v Leong (unreported) 10 April 1985 HC (No 272/1982) the court relying on evidence from a private investigator which implicated that the respondent and the correspondent had spent the night in the same house, found that on a balance of probability that the petitioner had established the commission of adultery by the respondent and co-respondent.