Economic migration has increased dramatically over the past decades, yet, still little is known about the well-being of economic migrants. Most studies focus on the poor mental health of migrants. The few studies that examine well-being do not include a comprehensive range of predictors. They focus mostly on socio-economic factors and a limited number of cognitive-behavioural factors. Furthermore, most research is not theoretically based. The present study aims to (1) examine a comprehensive range of determinants of well-being (i.e. circumstantial, cognitive-behavioural and personality) among economic migrants; (2) establish the extent to which the findings are supported by an integrative theory of well-being—the Sustainable Happiness Model (SHM). A total of 188 Polish immigrants living in Scotland took part in the study. Participants completed a survey on well-being and potential predictors: circumstantial factors (e.g. socio-demographics), cognitive-behavioural factors (e.g. coping strategies) and personality. The data were analysed by descriptive statistics, multiple and hierarchical regression. The results indicated that cognitive-behavioural factors account for most variance in well-being and personality explains the least variance. This contradicts the SHM which posits that well-being is mostly accounted for by the set-point. It was argued that the SHM, largely based on studies conducted on non-migrant populations in developed countries, may not be applicable in its current form to economic migrant populations or non-western cultures. A modified model is proposed to explain well-being of migrant populations and non-western, more collectivistic societies. The study highlights the importance of addressing migrants’ adverse circumstances to help maintain well-being.