Electrospun polyaniline (PAni) fibers doped with different levels of (+)-camphor-10-sulfonic acid (HCSA) are fabricated and evaluated as chemiresistive gas sensors. The experimental results, based on both sensitivity and response time, show that doped PAni fibers are excellent ammonia sensors and that undoped PAni fibers are excellent nitrogen dioxide sensors. The fibers exhibit changes in measured resistances up to 60-fold for ammonia sensing, and more than five orders of magnitude for nitrogen dioxide sensing, with characteristic response times on the order of one minute in both cases. A time-dependent reaction-diffusion model is used to extract physical parameters from fitting experimental sensor data. The model is then used to illustrate the selection of optimal material design parameters for gas sensing by nanofibers.