When a gas containing a condensable vapor expands from a reservoir through a subsonic or supersonic nozzle, the gas is cooled and the vapor can condense to form particles. A theoretical framework to describe such systems is given. Calculations based on the theory show that approximately uniform submicrometer particles can be produced at high mass rates by subsonic nozzle expansion. Four operating conditions (exit Mach number and the reservoir temperature, pressure, and saturation ratio) and the nozzle shape (area distribution and length) control the spread of the particle size distribution. Two modes of nozzle operation are predicted. In one case nucleation is confined to the nozzle while in the other case nucleation occurs both in the nozzle contraction and in a straight section following the nozzle. Nearly monodisperse particles can be produced for a wide range of operating conditions for both types of nozzle operation. Particle properties are less sensitive to operating conditions, however, when the conditions are chosen so that nucleation is confined to the nozzle. © 1987 American Institute of Physics.