The changes in the composition of the neutral gas in the space shuttle payload bay caused by the shuttle's vernier reaction control system rocket engines were measured with a quadrupole mass spectrometer aboard STS-4. The average hydrogen level during thruster firings was approximately 25 times greater than nitric oxide, NO, which was previously found to be one of the largest components of the gas in the payload bay during firings. The attitude of the orbiter also influenced the effects of the vernier engines as measured in the payload bay. When the mass spectrometer was pointed into the velocity vector, thrusters whose exhaust plumes were directed approximately upstream into the flow of ambient neutral species caused decreases in atomic oxygen signals. NO was enhanced in the payload bay during thruster firings primarily when the shuttle was in a bay-to-ram attitude. The return flux from the left-side, downward pointing vernier thruster was detected by the mass spectrometer only when the shuttle was in a belly-to-ram attitude. All of these results are consistent with earlier observations that the kinematics of gas phase and surface scattering are important in determining the effects of the thruster firings on the payload bay environment.