This research analyzed the influence of fat content (1%, 3%, 5%, and 10%, w.b.) and of water activity (0.85, 0.90, 0.95, and 0.99) on the effectiveness of microwave heating within model systems (agar gel). Findings indicate that an increased fat content promotes more uniform heating in the model systems and results in longer heating times to reach the target temperature of 50°C. In the model systems with different aw values, the warming of the central model zone was slower while in the corners, heating was accelerated. Additionally, in these models, heating times increased as the aw was decreased. PRACTICAL APPLICATION: The study of model systems allows for a detailed assessment of food when subjected to microwaves; and how heating is influenced by food properties such as aw and fat content. Findings indicate that changes in product formulation could improve the heating of ready-to-eat foods, either by increasing the fat content (better uniformity) or having high aw (shorter heating time).