In models of reactions occurring in atmospheric aerosol particles, calculated concentrations of some trace species often imply fractions of a molecule per particle. When this occurs, averaging rate or equilibrium expressions over a large number of particles gives results that differ from bulk solution. Two different effects are demonstrated here. In the first, the confinement of ions in submicrometer drops suppresses the dissociation of weak acids and bases relative to bulk solution. For free radicals, there is an isolation effect that suppresses radical chain termination reactions when radical concentrations are low. Under extreme non-equilibrium conditions, half the drops contain one radical, even when bulk solution calculations indicate a concentration orders of magnitude smaller. This can greatly enhance the rates of radical chain reactions in drops relative to bulk solution. This latter effect has the potential to dramatically alter modelled rates of reactions in atmospheric particles. ¿ 1997 American Geophysical Union