Sexus (1949), the first volume, describes the break-up of Miller's first marriage to Maude as he meets, falls in love with and marries his second wife, the captivating and mysterious dancer Mona (June). All the while, he feels guilty for leaving Maude, and becomes more attracted to her following their divorce. At the beginning of Sexus, Miller is 33 years old. June is at first called Mara, but at the beginning of chapter 8, and for the remainder of the trilogy, her name is changed to Mona. Miller states that this is under the influence of his friend Dr. Kronski, and that the name change accompanied "other, more significant changes." She is one who has changed many details of her life: "her name, her birthplace, her mother, her upbringing, her friends, her tastes, even her desires."[2]

The New York Times stated, "Miller uses licentious sex scenes to set the stage for his philosophical discussions of self, love, marriage and happiness."[3]

Miller said that, in a burst of inspiration one night in 1927, he stayed up all night plotting out Tropic of Capricorn (1939) and The Rosy Crucifixion in forty or fifty typewritten pages. He began writing Sexus in New York in 1942, then set it aside until picking it back up in 1947 while living in Big Sur. It was first put out in Paris as two volumes by Obelisk Press in 1949. It created a big stir, and was banned the following year, with the publisher fined and given a prison sentence.