A mathematical model for an autothermal reforming (ATR) fuel processor has been developed to study the transient response of the reactor for gasoline reforming. The model was developed with experimental data from temperature profiles along the reactor and product composition (CO2, CO, and H-2 formation). Model versus experiment results were compared for three cases: partial oxidation, ATR using steam injection, and ATR using liquid water spray. In addition to analysis capabilities, the developed model is intended to help in the design of a feedback controller aimed at improving the dynamic response time of the system.