Codes for the prediction of spacecraft environments are currently in widespread use; however, few if any comparisons of code results with experimental measurements have been performed. In this paper we report the results of calculations performed for the Atmosphere Explorer-D satellite at an altitude of approximately 250 km. Measurements of the atmosperic composition at the ram surface of the spacecraft were obtained by a closed source mass spectrometer before and during the operation of a return flux experiment in which a neon plume was directed into the spacecraft velocity vector. Calculations using the Integrated Spacecraft Environment Model (ISEM) reproduced the measured fluxe of the ambient species immediately before and during the venting operation. The prescence of the neon plume reduced the flux of ambient species reaching the detector. The model reproduced these trends and gave a range of calculated neon return flux with a mean approximately equal to the value derived from the measurements. The model predicted a surface density of ambient species approximately 33 times the ambient level with and without the plume and nonnegligible scattering of neon into the wake of the spacecraft during plume operation. ¿ American Geophysical Union 1989