Bariatric surgery is the most effective treatment for severe obesity. However, following Roux-en-Y gastric bypass (RYGB) surgery, a small minority of patients develop new-onset alcohol use disorder (AUD), the aetiology of which is poorly understood.The aim is to construct a theory to explain the development of AUD among a sample of individuals who reported problematic drinking following RYGB.Semi-structured interviews were conducted with eight RYGB patients diagnosed with AUD attending a multi-disciplinary outpatient weight management service at a public hospital in the Republic of Ireland. A constructivist grounded theory methodology was used to analyse interview transcripts.Participants' main concern was identified as 'unresolved psychological issues' which were managed by 'external coping mechanisms', namely, 'eating to cope'. After RYGB, comfort eating was no longer possible to the same extent. Following a 'honeymoon period', participants' need for an external coping mechanism resurfaced. 'Filling the void' provides a framework to explain how participants managed the symptoms of their unresolved psychological issues through 'behavioural substitution', that is, drinking alcohol instead of eating.The theoretical framework of 'filling the void' adds to contemporary research that conceptualises AUD behavioural substitution as 'addiction transfer' by describing the process by which the phenomenon occurs as well as the characteristics of participants. The clinical implication of this research is to advocate for a reshaping of treatment of RYGB patients, with increased psychological input following surgery.