In Act III, Scene I, both Cecily and Gwendolen confront Jack and Algernon as they enter from the garden. Cecily demands to know why Algernon pretended to be Jack's brother. When he answers that it is to have an opportunity for meeting her, she is thrilled with the 'wonderful beauty of his answer,' despite her suspicions that he could be lying.

Gwendolen also confronts Jack about his pretensions of having a brother. Jack gives a similar answer to the one given by Algernon. Although both women are skeptical of the men's true sincerity in their avowals, Gwendolen declares that she is not about to give in to 'German skepticism' and Cecily contends that Algernon's voice alone inspires his pronouncements with 'absolute credulity.'

When both men further declare that they will be rechristened 'Ernest' that very afternoon, both women are persuaded that the 'self-sacrifice' and the 'physical courage' exhibited by the male species eclipses that of the female's. As the couples embrace, Lady Bracknell enters. She is shocked by the boldness exhibited by the amorous young people. When she demands to know the meaning of the affectionate displays, Gwendolen admits that she is engaged to Jack. Despite both Jack and Gwendolen's assertions, Lady Bracknell refuses to acknowledge the engagement.

She turns instead to Algernon and asks about Mr. Bunbury and whether he lives in the house. Algernon, to his great embarrassment, stammers that Bunbury is quite dead, having died after the medical assessment of his doctors that 'Bunbury could not live.' Next, Lady Bracknell appears to turn her attention to Cecily. At this point, Jack announces that his ward is engaged to Lady Bracknell's nephew, Algernon. Lady Bracknell is not impressed with the announcement, as she terms the undue number of engagements in 'this particular part of Hertfordshire' as 'considerably above the proper average that statistics have laid down for our guidance.'

Lady Bracknell and Jack further discuss Cecily's heritage and ancestry at some length. Although Jack produces certificates of Cecily's 'birth, baptism, whooping cough, registration, vaccination, confirmation, and the measles; both the German and the English variety,' Lady Bracknell only takes notice of Cecily when she realizes that the young lady is in possession of 'a hundred and thirty thousand pounds in the Funds.' On realizing Cecily's material advantages, Lady Bracknell is suddenly in favor of a short engagement between Cecily and Algernon, reversing her initial decision against any marriage plans for the couple.

Meanwhile, Jack absolutely refuses to give his consent to Cecily's engagement. He tells Lady Bracknell that the engagement is 'quite out of the question' and that Cecily may not marry without his consent until she is of age. As of the moment, Jack refuses to give his consent at all. He cites defects in Algernon's moral character, his tendency to untruthfulness. Jack further states that the terms of her grandfather's will states that Cecily will not come of age until the age of thirty-five. Here, Algernon passionately vows that he will wait for Cecily if he needs to, but Cecily is not enthusiastic about waiting.

This is where Jack chimes in that he will consent to his ward's marriage to Lady Bracknell's nephew if Lady Bracknell would consent to his marriage with Gwendolen. This is the only condition by which Jack will consent to his ward's marriage. As Lady Bracknell is non-committal, Jack dramatically asserts that 'a passionate celibacy is all that any of us can look forward to.' Lady Bracknell responds that, although she does not wish such a fate for her daughter, her nephew may choose for himself.