Problem-solving is an essential transversal and learning skill in the 21st century. To better support it, we need a deeper understanding of its processes, i.e., problem exploration, problem representation and knowledge use. Understanding the mechanism behind the transitions between these processes plays an important role in supporting its development. With a sample of N = 1828 firstyear university students, this study is the first to measure how students transition between the different problem-solving processes and how this influences their overall problem-solving performance as well as how this is related to their other test-taking behaviours. Results indicate that mastering the first transition - correctly understanding and depicting the problem structure - plays a crucial role in problem-solving. Students who failed to master it regularly failed in the next transition. Problem complexity strongly influenced transitions during the entire problemsolving process. Based on students' behavioural patterns, we distinguished four qualitatively different latent transition classes: expert transitioners, advanced transitioners, beginner transitioners and non-transitioners. The number of interactions proved to be a more effective profile characteristic, especially on high-complexity problems, than time spent on the problem-solving process. The results of the current study provide important insights into how students' transition between the different problem-solving processes and how their test-taking behaviour indicates their transition skills.