This article analyses rare surviving adulterous love letters alongside published epistles and trial reports to reveal the practical and emotional importance of letter-writing in conducting an affair in England c.1740–1830. While attitudes to adultery have received widespread scholarly attention, illicit letters remain largely overlooked. The article is the first to outline distinguishing features of adulterous letters, and the language of infidelity. It distinguishes missives from courtship letters as a secretive genre carefully shielded by writers. By scrutinizing the letters which sustained affairs, the article rediscovers the happiness, jealousy and desire of illicit love in the words of lovers themselves. [Open Access]