The aim of this study was to reveal 5th class middle school students' determination skills on the truth value of mathematical propositions about multiplication and division. These skills were evaluated in the context of operation meanings preferred and arguments constructed by the students in the process. The research group consisted of 95 fifth-grade students studying at a state middle school in Turkey. The study an example of sequential explanatory from mixed research designs. The data of the study were collected in three stages. In the first and the second stage, quantitative data of the study were gathered with Awareness Questionnaire (AQ) and Proposition Evaluation Test (PET) developed by the researchers. In the third stage, semi-structured task-based interviews were conducted. As a result of the study, it was revealed that students had more difficulty in selecting false mathematical propositions. Most of the students used counterexample and deductive warrants for the falsity of the propositions, and inductive warrants for the truth of the propositions. These results showed that determining the false propositions was an important ability and students preferring operational meanings had disadvantages in this process.