Internationalization is complex process with various challenges arising at various stages requiring according solutions. Firms struggle to understand at what stage they are, what their degree of internationalization is, and what to do next. Internationalization is product related activity. Firms internationalize their product offer. The pattern of product portfolio formation can tell more about the past of the firm and precondition future. Yet, behavioral internationalization theories do not consider products as part of internationalization decision-making due to the limited perspectives applied for theory development. Academia proposes tactical solutions to otherwise strategic challenges limiting the use of theories to initial internationalization engagements. Given the problem, the thesis looks at the pattern of product portfolio formation along the entire internationalization process. This work helps identify the degree of internationalization along internationalization stages. This work proposes the development of POM-model for development of internationalization strategies from firm-level perspective. The study takes interpretivist philosophic paradigm in observing process of internationalization and product portfolio formation from process theory perspective with application of process data collection method, in particular, the visual mapping strategy. The key contributions of the study are proposition of the classification of firms according to their internationalization effort and the theoretical framework which helps firms identify the degree of internationalization by analysis of the pattern of product portfolio formation along the stages of internationalization.