This thesis sets in dialogue concepts from Decadent and Adaptation Studies within the arena of Huysmans’ novel À rebours as an extended case study. Examining this text as and containing versions of what might be argued to be adaptation, the research explores border zones of contemporary Adaptation Studies, using this as an alternate approach to examining specific types of intertextuality within this novel as the ‘breviary of the Decadence.’ In finding conceptual inherencies between Decadent themes and aspects of adaptation, and considering Huysmans’ own preoccupations through the framework of his oeuvre and biography, an argument is proposed which reads the adaptations in À rebours as detailing Huysmans’ experiment with Decadence. À rebours is posited as both being, containing, and allegorising adaptation(s) which are defined by and a part of Decadent aesthetics. Ideas such as artifice, ornamentation, decay, curation, the mise-en-scène, the memory palace, entropy, and embodiment all contribute to exploring what adaptation means for the Decadent figures of author and character in À rebours. The adaptations proposed and identified diversify and add to the repository of potential forms of adaptation, as well as providing new conceptual models for particular versions and aspects of adaptation. The work examines adaptation avant la lettre in a specific aesthetic and authorial context, and tests current and new methodologies for the study of adaptation, whilst expanding the terminology for how Adaptation Studies theorises adaptation, adaptations, their reception, and their significance for adapters.