The Particle Analysis by Laser Mass Spectrometry (PALMS) instrument was used to obtain the first in situ measurements of the composition of particles in the wakes of solid rocket motor (SRMs) launch vehicles. PALMS acquired mass spectra of over 2300 exhaust particles within the plumes of an Athena II rocket and the Space Shuttle. The majority of positive spectra indicated the presence of primary and trace components of the aluminum fuel and the combustion catalyst. Negative spectra showed chlorine from the oxidizer. Nitrate and phosphate fragments and water were common features of spectra acquired during the Space Shuttle encounters. Elemental carbon (EC) was a significant particle type observed in the Athena II plume. The data show that particles emitted by SRMs are more diverse and probably more reactive than previously considered.