A system can be customized by is owner. The fundamental premise behind designing for customization is that it improves the user experience (UX) of the system. In this study, we contend that the effects of customization on UX of a smartphone can be theoretically modelled as users’ beliefs about the system object (customization) that influence their attitudes towards the system object (perceived system usa-bility), which in turn shapes their beliefs (flow) and attitudes (engagement) towards using the system. We tested this proposition via an experimental study with 50 college students as participants. Each participant was asked to perform customization tasks on a smartphone, and then instructed to com-plete a comparison task aimed at contrasting customized user interface with a standard one. Our ma-nipulation checks confirmed that the customization task, in particular, the customization of the layout was more pronounced for participants. Analytical results from the comparison reveal that customiza-tion positively influence users’ evaluation of three key constructs of UX: perceived system usability, flow and engagement, and that the feeling of engagement is mediated through perceived system usa-bility and flow. We conclude with a discussion of the impact of customization on UX, and whether the distinction between object- and behaviour-based beliefs and attitudes is helpful.