How do reasoners negate compound sentences, such as conjunctions of the form A and B and disjunctions of the form A or B or both? A theory based on mental models posits that reasoners negate each clause independently, and enumerate the various possibilities consistent with the negation. It makes a novel prediction: negations of conjunctions should be more difficult to comprehend than negations of disjunctions. Two experiments corroborate the prediction. Experiment 1 tested participants’ ability to comprehend sentential negations by giving them assertions of the form: Bob denied that he wore a yellow shirt and he wore blue pants on Tuesday. Participants selected the clothing options that Bob possibly wore on Tuesday. Experiment 2 gave participants descriptions such as Bob loves Mary or Mary loves John or both, and they were required to formulate a denial by completing a sentence that started with “No, ...”. In both studies, participants’ responses were more accurate for denials of disjunctions than denials of conjunctions.