The freezing of deliquesced aerosols is the most likely formation mechanism of ice particles in wave clouds. A similar microphysical process has been postulated for the formation of aircraft contrails. Aerosol and cloud particle measurements have been made in contrail and wave clouds that formed under similar environmental conditions. Microphysical and optical properties are compared for these two types of clouds using a number of unique capabilities of the MASP particle probe, i.e., direct detection of forward and backscattered light, and the measurement of particle spacing to determine small scale cloud structures relevant to microphysical processes. The data indicate similarities in optical properties, but significant differences in the number concentrations, liquid water contents and the spatial organization of the cloud particles in the two types of clouds. ¿ 1998 American Geophysical Union