This paper studies the cognitive obstacles related to one aspect of mathematization in physics problem-solving, namely, what might be called "structuring for mathematization," where the problem situation is structured in such a way that a translation to a mathematical universe can be done. We report the results of an analysis of four protocols from task-based interviews with university students working on physics problems in pairs who fail when solving the problems. The obstacles encountered by the students are shown to be related to how the students approach the structuring for mathematization, but they are met at different steps in this process. Four types of obstacles are identified.