7-Phenylacetic acid-4-alkyl-coumarinyl amides having the formula ##STR1## wherein R is alkyl having 1 to 4, in particular 1 to 2 carbon atoms, optionally substituted by one or more halogen atoms, in particular fluorine atoms, especially methyl or trifluoromethyl, are suitable substances for the fluorometric determination of the activity of hydrolases, in particular of penicillin G-acylase, since by the enzymatic reaction they provide cleavage products which in regard to their fluorescence properties differ drastically from those of the substrate. They can be used in processes for the fluorometric determination of the activity of hydrolases, in particular of penicillin G-acylase.