The aim of this study is to describe some key components of the relation between reasoning and gaze behaviour. Specifically, we studied gaze dwell and fixations during a sentence-equivalence task that required the processing of compound negation for conjunctions and disjunctions. We derived opposite predictions from two different theories of reasoning. The Mental Models Theory was tested against the formalist PSYCOP theory. The former predicts specific patterns of inspection times and gaze fixations frequency as functions of representational complexity and semantic depth. That is, the more complex representation is, and the deeper the meaning processing is, the longer inspection times and the higher fixations frequency should be. PSYCOP predicts the opposite visual behaviour pattern or no difference at all. We tested such predictions experimentally using eye-tracking technology. Our results support the Mental Models Theory. That is, it seems that human mind constructs mental models to process the negation of conjunctions and disjunctions according to eye-tracking evidence.