Submicron flyash particles show a preferential surface concentration of sulfur and chlorine in comparison to their matrix elements, e.g., aluminum and silicon. The relationship between concentration of surface deposited matter and inverse particle diameter suggests that S and Cl are adsorbed and oxidized on flyash particles after volatilization in the high-temperature zone of the boiler. This leads to the formation of cloud nuclei even before a sulfur gas-to-particle conversion takes place.