A spherically symmetric quasi-steady model has been formulated for droplet combustion which includes complex chemistry and variable properties. The model is applied to mixtures of heptane and water. Calculations from the model show that the acetylene concentration inside a heptane flame increases with increasing droplet diameter. This trend suggests an increased propensity for soot formation with increasing droplet diameter which is in agreement with some qualitative trends from experiment. Predicted quasi-steady burning rate constants for pure heptane droplets, however, do not vary with droplet diameter which may be attributed to the model＇s inability to account for the physical presence of soot particles and aggregates and their effect on heat transfer to the droplet. The addition of water to heptane is found to lower burning rates and C2H2 concentrations, and a limiting water concentration for stable burning is predicted, similar to prior reported analyses based on a simplified one-step reaction mechanism.