Rapid changes in photoluminescence (PL) intensity and blue shifts of typically 100 nm have been measured for porous silicon wetted by hydrofluoric acid. The differences between continuous and interrupted PL excitation show that the process is not solely limited by photoactivation. In contrast, the effect of potassium hydroxide, a second silicon etchant, is to reduce PL intensity without any significant blue shift. These experiments show the importance of surface chemistry in PL of wet porous silicon and are interpreted in terms of surface termination by different species.