Fully CMOS-compatible, surface-micromachined polysilicon microbridges have ben designed, fabricated, and tested for use in catalytic, calorimetric gas sensing. To improve sensor behavior, extensive electro-thermal modeling efforts were undertaken using SPICE. The validity of the SPICE model was verified by comparing its simulated behavior with experimental results. The temperature distribution of an electrically-heated microbridge was measured using an IR microscope. Comparisons among the measured distribution, the SPICE simulation, and distributions obtained by analytical methods show that heating at the ends of a microbridge has important implications for device response. Additional comparisons between measured and simulated current-voltage characteristics, as well as transient response characteristics, further support the accuracy of the model. A major benefit of electro-thermal modeling with SPICE is the ability to simultaneously simulate the behavior of a device and its control/sensing electronics. Results for the combination of a unique constant-resistance control circuit and microbridge gas sensor ar given. Models of in situ techniques for monitoring catalyst deposition are shown to be in agreement with experiment. Finally, simulated chemical response of the detector is compared with the data, and methods of improving response through modifications in bridge geometry are predicted.