A combined optical and particle collection probe which employed an in situ particle size distribution measurement method and an infrared camera were used to obtain data that could be used to validate solid propellant rocket motor plume signature prediction codes. The probe design was optimized and the required rates for window purge and ejector flows were determined which provided proper functioning of the probe when place in the supersonic region of the plume. At the nozzle exit the particle size distribution was quadramodal with most particles smaller than 0.5 micron and the largest particles with diameters less than 25 microns. The larger particles at the nozzle exhaust were not present further aft in the plume. This behavior was believed to result from the breakup of the large Al2O3 particles in the plume Mach discs. However, further work is required to determine if the probe alters the particle size distribution.