In the United States, eighty percent of the adult homeless population smokes cigarettes compared to 15 percent of the general population. In 2017 Power to Quit 2 (PTQ2), a randomized clinical trial, was implemented in two urban homeless shelters in the Upper Midwest to address concurrent smoking cessation and alcohol treatment among people experiencing homelessness. A subset of this study population were interviewed to assess their experiences of study intervention. The objective of this study was to use participants' experiences with the intervention to inform future implementation efforts of combined smoking cessation and alcohol abstinence interventions, guided by the Consolidated Framework for Implementation Research (CFIR). Methods: Qualitative semi-structured interviews were conducted with 40 PTQ2 participants between 2016-2017 and analyzed in 2019. Interviews were audio-recorded, transcribed, and analyzed using a socially constructivist approach to grounded theory. Findings: Participants described the PTQ2 intervention in positive terms. Participants valued the opportunity to obtain both counseling and nicotine-replacement therapy products (intervention characteristics) and described forming a bond with the PTQ2 staff and reliance on them for emotional support and encouragement (characteristics of individuals). However, the culture of alcohol use and cigarette smoking around the shelter environment presented a serious challenge (outer setting). The study setting and the multiple competing needs of participants were reported as the most challenging barriers to implementation (implementation process). Conclusion: The CFIR was useful in offering specic insights about the implementation context of the PTQ2 intervention, including the impact of the factors in the inner and outer settings of the shelter environment. Participants described a discord in their personal goals of reduction compared with the study goals of complete abstinence, which may pose a challenge to the ways in which success is dened for people experiencing homelessness. Contributions To The Literature People experiencing homelessness have worryingly high rates of smoking, but do express motivation to quit smoking. However, intervention adaptation is required to effectively address this population's unique needs. This study addresses the lack of research on the implementation of smoking cessation interventions that target people experiencing homelessness, despite the mixed success of such programs on cessation outcomes. To our knowledge, this is the rst time that CFIR has been applied to the implementation of a smoking and alcohol cessation intervention for smokers experiencing homelessness.