We report the desorption characteristics of H2O, CO2, H2 and O2, and changes in electrical resistance, single walled carbon nanotube (SWNT) mats during vacuum annealing from room temperature to 873 K. H2O desorption at ~503 K coincides with a resistance decrease of ~ 2%, in agreement with theoretical calculations. CO2 and H2 desorption correspond to subsequent decreases in resistance at ~563, and 663 K respectively. Repeated gas exposures, after thermal desorption, result in reversible R-T characteristics, indicating the tunability of average electrical response of SWNT mats. Our results showing the lack of O2 desorption, and increased CO2 desorption after O2 exposure, show that oxygen is strongly chemisorbed to SWNT surfaces, and desorbs primarily as CO2 and CO. These results are of relevance for harnessing SWNT-based nano-structures for potential applications such as hydrogen storage and chemical sensors, where the resistance-temperature characteristics and gas adsorption phenomena are intimately related.