This paper explores extramarital aﬀair as a ground for civil divorce in the practice of courts of the Duchy of Warsaw. In 1808 the Napoleonic Code was enforced on this territory. Under the rules of the Code Civil a couple could petition for a divorce with mutual consent (article 233). There were also three grounds entitling either spouse to demand a divorce, when it was possible to attribute fault to the second party. In the case of the husband’s inﬁdelity, the wife could demand a divorce only when he brought his concubine into their common residence (article 230). On the other hand, the husband could demand a divorce as a result of every act of his wife’s adultery (article 229). Moreover, each spouse could demand a divorce for outrageous conduct, ill-usage or grievous injuries exercised by the other (article 231). Condemnation of one of the married parties to infamous punishment was also a ground for a divorce to the second party (article 232). In spite of the discrimination of women in cases of adultery, more than 75% of all divorce judgments were issued as a result of actions ﬁled by wives. This was probably caused by the Polish Courts which applied a broad interpretation to article 230 in their jurisprudence. Each act of a husbands’ adultery in a common residence was treated as a ground for divorce, although, according to article 230, a wife had to prove that her husband lived with his concubine in a conjugal home. The husbands’ adultery can be easily distinguished from that of wives’. Generally, men committed adultery out of lust. Female inﬁdelity was usually connected with leaving a husband. Husbands often committed adultery with maids, whereas wives – with soldiers.