A quantitative study of the hydrolytic dissolution of solid p-chloranil at alkaline pH using a combination of the channel flow cell and in situ atomic force microscopy measurements shows that the dissolution is driven by reaction of the substrate with hydroxide ions at, or very close to, the dissolving surface. The dissolution rate equation, deduced from the channel flow cell for single crystals of p-chloranil, is rate/mol cm(-2) s(-1) = 3.8 X 10(-7)[OH-](0) where [OH-](0)/M is the hydroxide ion concentration adjacent to the solid surface. An analogous rate law is derived for pressed pellet substrates. The novel application of the atomic force microscope to make absolute measurements of surface averaged dissolution rates is described. Comparison with the independent channel flow cell data reveals good mechanistic agreement and consistency of rate constants provided it is recognised that a thick stagnant diffusion layer can be present under the usual conditions employed for AFM.