The formation rate of porous silicon by photoelectrochemical etching of n-type silicon is measured in situ by a novel technique. The reflection of the laser beam used to drive the reaction contains circular patterns, the outer radius of which bears a linear relationship to the depth of etching. The use of the radius to measure etch rate and the absolute depth of etching is described and justified. The rates are analyzed in tenus of the calculated activities of the different species in solution. A rate equation, which is first order in HF and HF2-, is deduced and rate coefficients calculated. It is concluded that the etching mechanism includes two parallel rate-determining steps involving HF and HF2- and that the HF2-channel is roughly 20 times faster than the HF channel. In light of our data, previously proposed mechanisms of Si dissolution in fluoride solutions should be reevaluated.