The paper discusses integrating travel demand forecasting models with the Geographic Information System (GIS) to estimate hot stabilized mobile source emissions. In developing a regional mobile source emissions model using a GIS framework, the emissions model is designed to improve emission estimates by accounting for the spatial and temporal effects of a variety of vehicle activities, environmental factors, and vehicle and driver characteristics. While a description of the overall modeling approach is given, the emphasis of the paper is to describe the hot stabilized emissions estimation process and the role of travel demand forecasting models. Although travel demand forecasting models were designed for predicting future capacity requirements, they also provide useful information needed for mobile source emissions estimates. Improvements to travel demand forecasting models to more accurately predict hot stabilized emissions are also discussed.