Research on the electronic applications of organic semiconductors is growing rapidly, and both polymeric and small molecule organics are being used in the fabrication of LEDs, transistors, and biological/chemical sensors. As a part of the chemical microsensor project at NIST, we are exploring various organic semiconductors for application to chemical gas sensing. In this study, the electronic effects produced by controlled gas exposures on a-sexithiophene (a6T) thin films have been investigated using x-ray and ultraviolet photoelectron spectroscopy (XPS and UPS). Incremental exposures of a6T films to NO2, O2, NH3 in N2, and water vapor in N2 were performed in ultra-high vacuum, while repeated doses of O2, H2 in N2, and methanol vapor in N2 were performed ex situ at ambient pressure. In both cases XPS spectra of gas-exposed films showed no evidence of chemical changes. However, the features in both XPS and UPS spectra were observed to shift as a whole, roughly in proportion to the total gas dosage, and to a greater or lesser extent depending on the specific gas. These effects could be reversed by gently heating the films to temperatures around 100C. This behavior is interpreted in terms of doping by weakly bonded gas species within the near-surface region of the a6T films. Greater doping effects were observed for films dosed at ambient pressure. We discuss possible gas adsorption models that may explain the differing gas sensitivities and their dependencies on pressure, temperature, and exposure time. Finally, temperature programmed desorption was used to study the reversibility and resistance to reaction of a6T films during this gas adsorption/desorption process. The organic films were found to be chemically stable to gas exposure and subsequent thermal desorption in the dosage ranges explored.