In order to study the gaseous environment of the STS orbiter a mass spectrometer was operated aboard the SPAS-01 subsatellite on the shuttle missions STS 7 and 41-B. Data are reported on the impact of the operation of attitude control thrusters on the atmosphere in the payload bay. Firings of aft mounted thrusters have been observed to increase the total number density inside the payload bay to more than 1018 m-3 (equivalent to more than 7¿10-5 mbar), whereas the operation of most of the forward mounted thrusters yields hardly a detectable signal. Nitric oxide is a major gaseous constituent inside the payload bay during the operation of aft mounted thrusters. Water desorbing from surfaces leads to a slowly decaying total number density following engine shutdown.