Sapote mamey is a sweet and aromatic tropical fruit that is very perishable. It is a climacteric fruit and has high rates of respiration and ethylene production. Maturity indices for this commodity are difficult to define because fruit show few changes in external appearance as they ripen. The fruit flesh, however, shows large changes in color, firmness, and sugar content measured as soluble solids content (SSC). The objective was to model fruit ripeness from measurements of SSC. We selected SSC because it is easy to measure and because sweetness is an important quality attribute in sapote mamey. Typical values of SSC range from 12% (immediately after harvest) to 30% to 35% (ripe fruit). A linear-plateau model was used to describe the changes in SSC over time of ripening fruit kept at different temperatures. The model assumed that, as fruit ripened, SSC increased at a linear rate reaching a maximum of 30% SSC at the ripe stage after which SSC changed little. From the model we calculated the rate of fruit ripening and the time to reach the ripening stage (30% SSC). The rate of ripening showed a quadratic relationship with storage temperature. Fruit kept at 27, 25, or 20 °C ripened 3.5, 5, or 7 days after harvest. The model can be used to estimate when fruit will reach the ripe stage, as long as we know the initial SSC and storage temperature. This model was constructed from data obtained over 2 years from fruit grown in the state of Morelos, Mexico. It is still to be tested for its applicability on fruit from other growing regions.