The central goal of this study was to analyze the complexity of students' explanations about how and why chemical reactions happen in terms of the types of causal connections students built between expressed concepts and ideas. We were particularly interested in characterizing differences in the types of reasoning applied by students with different levels of training in the chemistry, from college to graduate school. Using a qualitative research approach, we identified diverse modes of reasoning expressed by students when engaged in the analysis of different sets of chemical reactions selected to produce a targeted compound. Main findings indicate that dominant modes of reasoning varied with educational level and the nature of the task. Although participants applied diverse modes of reasoning, linear causal reasoning was prevalent across educational levels and types of tasks. Many students tended to generate explanations based on the identification of a single agent that caused a sequential chain of events. Advanced undergraduate students in our sample generated the most complex explanations. The results of our study have important implications for the development of causal mechanistic reasoning in chemistry.