Several investigators have reported that constant aerosol surface areas per volume of gas are established when aerosols are generated by chemical reaction at a constant mass (or volume) rate. A simple relationship between these surface areas and the rate of gas-to-particle conversion has been observed over a wide range of aerosol formation rates. In this paper, a theory is developed which explains these observations. For monodisperse aerosols and aerosols whose size distributions are self-preserving, the surface area per unit volume of gas can be expressed as a function of the rate of gas-to-particle conversion, dV dt, and time, t. For the special case of constant dV dt, surface area varies as dV dt 3 5t 1 5 and is essentially independent of the chemical nature of the secondary aerosol. The values and functional dependences of measured surface areas are in good agreement with the theory given in this paper. Thus by carrying out photochemical reactions under controlled conditions, aerosols of predictable surface area can be generated.