Background: We tested the effects of an intervention on the learning of introductory thermodynamics principles. This intervention, OEM-Thermo, is designed to prompt the cognitive operations of meaningful learning: organization, elaboration, and monitoring. We also sought evidence to show that execution of these operations was associated with learning gains and that cognitive operations are influenced by different intervention exercises. Purpose/Hypothesis: Study 1: Students who complete OEM-Thermo will gain more conceptual knowledge than students who complete traditional problems. Study 2: First, elaboration and monitoring contribute to learning with OEM-Thermo. Second, students engage in elaboration and monitoring at a higher rate when answering elaboration questions than when completing matrix exercises. Design/Method: Study 1: A two-group, pre- and post-test experimental design tested OEM-Thermo effectiveness. Study 2: A one-group, pre- and post-test design where participants thought aloud while completing OEM-Thermo tested deep and surface reasoning as well as the frequency of elaboration and monitoring events. Results: Study 1: A significant interaction between test time (pre- and post-test) and condition shows that OEM-Thermo promoted development of conceptual reasoning more effectively than did traditional homework problems. Study 2: Significant partial correlations were found between post-test scores on one of two deep reasoning categories and the frequency of elaboration and monitoring events in the think-aloud protocols. Differences were also found in the rate of elaboration across intervention exercises. Conclusions: An intervention that includes tasks designed to stimulate the cognitive operations of meaningful learning improves students' conceptual reasoning.