The relative importance of Brownian and gravitational coagulation on the transient behavior of an aerosol undergoing coagulation and gravitational settling is discussed. A comparison is given of the particle size distribution and settling rates calculated for a typical high-mass density aerosol using a gravitational coagulation capture efficiency, э, of unity, an э calculated as a function of particle radius, and an э of zero. It is concluded that the use of э = 1.0 predicts a non-log-normal size distribution and seriously overestimates the aerosol deposition rate, particularly at short times after the onset of settling.