Numerous field studies have shown that gas-to-particle conversion can be an important source of submicron secondary aerosol in power plant plumes. In this paper, aerosol size distribution data from several field studies have been analyzed to determine particle diameter growth rates by gas-to-particle conversion. The functional dependencies of these growth rates on particle diameter were then compared with theoretical models (i.e. growth laws) to infer possible chemical mechanisms of secondary aerosol formation. Based on this analysis it has been possible to infer the relative contributions of various postulated conversion mechanisms to the aerosol volume formation rate. It has been concluded that the chemical mechanisms of gas-to-particle conversion in power plant plumes depend upon ambient conditions. For the limited set of data analyzed in the paper, aerosol growth resulting from condensation of molecules formed by gas phase reactions was predominant in all cases. For plumes mixing into humid air (≥ 50%r.h.), however, heterogeneous reactions (e.g. oxidation of dissolved SO2 in droplets) accounted for up to 20 % of the aerosol volume formation rate. It is likely that at higher humidities, droplet phase reactions would be still more important.