4-Alkyl- and 7-alkoxycoumarin derivatives have been investigated for their spectroscopic properties in three different solvents. The substituents are methyl, ethyl, propyl and butyl groups. Absorption and emission spectra, quantum yields and mean fluorescence decay times have been determined. The spectroscopic studies together with calculations of the radiative transition probabilities indicate that the 4-alkyl- and 7-alkoxy substituents do not change the symmetry of the electronic cloud of the coumarin skeleton. The changes observed for the rates of radiative transitions and also for the positions of the absorption and emission maxima can be explained by changes in the molecular electronic transition moment, caused by the weak donation potential of the substituents. A strong dependence of the quantum yields and mean fluorescence decay times on solvent viscosity has been found, leading to the suggestion that torsional rotations of the alkyl and alkoxy substituents significantly increase the internal non-radiative energy conversion rate.