Spectral properties of coumarin derivatives (2H-1-benzopyran-2-one), substituted with a bulky group in position 3, were investigated in solvents and in polymer matrices. The bulky electron donating groups in position 3 were phenyl-, phenyltio-, 2-methylphenyltio-, 2,6-dimethylphenyltio-, benzyl-, phenoxy-, dimethylamino- and benzoylamino-. The absorption spectra of all the derivatives are dominated by a broad band with a maximum at 340 nm (log epsilon similar to 4.0), which were not influenced by the polarity or viscosity of the environment. The fluorescence of these derivatives is strongly influenced by the polarity of the solvent and viscosity of the surroundings. In high viscosity solvents and in polymer matrices, the quantum yields of around 0.1 and a lifetime of around 2 ns was observed. In low viscosity non-polar solvents such as cyclohexane, the quantum yields lower than 0.01 were observed. The fluorescence of coumarin probes was quenched by polar methanol with a bimolecular rate constant, k(q), larger than diffusion controlled limit indicating static quenching. The increased polarity of the mixed solvent introduces processes such as intramolecular charge transfer or twisted intramolecular charge transfer which effectively compete with fluorescence. The dependence of quantum yield of fluorescence on temperature was determined in viscose solvents and polymer matrices. The activation energy of radiation-less process (E-a) increased in going from phenyl to more the bulky 2,6-dimethylphenyltio group in non-polar high viscosity polybutene oil and polar glycerol supporting the idea that the radiation-less process is related to rotation of the group in position 3. The E-a value is lower in rubbery matrices such as polyoctenamer or atactic polypropylene than in glassy polymers such as polystyrene, poly(methyl methacrylate) or polyvinyl chloride. 3-Phenylcoumarin, due to its spectral properties, seems to be the most suitable probe for monitoring microviscosity in polymers.