Photolabile coumarinylmethyl esters of biomolecules (caged compounds) are new tools for studying spatial and time-dependent aspects of signal transduction in living cells. Herein we describe a fluoresence spectroscopic method for the determination of the rate constants of the photolysis steps of such caged compounds using (6.7-dimethoxycoumarin-4-yl)methyl diethyl phosphate (DMCM-DEP) and sodium (6,7-dimethoxycoumarin4-yl)methyl sulfate (DMCM-S). DMCM-DEP and DMCM-S are caged compounds which photorelease a proton, the corresponding acid anion, and the strongly fluorescent alcohol DMCM-OH upon excitation. The results of stationary and time-resolved measurements of the photochemistry and the luminescence of both caged compounds indicate that DMCM-OH is produced already during the excitation pulse. The quantitative analysis of the data demonstrates that the first step of the reaction-heterolytic bond cleavage of the coumarinylmethyl ester leading to the ion pair of a DMCM cation and an acid anion-is very fast with a rate constant of k(1) approximate to 2 x 1 10(10) s(-1). Recombination of the ion pair occurs with a rate constant of k(rec) approximate to 2.3 x 10(9) s(-1) and is about 10 times faster than the competing hydrolysis reaction of the DMCM cation yielding DMCM-OH and a proton. Thus, both caged compounds belong to the fastest phototriggers known.