It was found experimentally that free bodies 40 to 60 μm in size come into additional motion in various gases once a topochemical reaction has begun on their surface to yield a crystalline product on the surface or in the bulk. This motion was called chemojet motion, and the body's velocity was found to depend on the crystallization rate and to what extent crystallization is localized at active sites. Comparing the observed data with the mathematical model of chemojet motion suggested that this kind of motion is intermediate between Brownian motion and jet propulsion. Chemojet motion may be either directional or chaotic. Investigating this phenomenon is expected to deepen insight into surface processes.