Fluorescence spectra, fluorescence quantum yields and decay times were measured for fluorophores capable of either conformational or rotational mobility in dilute solutions of benzene, methyl methacrylate (MMA) and in the rigid poly(methyl methacrylate) (PMMA) matrix at room temperature. When passing from fluid solutions of MMA to the polymeric matrix of PMMA a dramatic increase in the fluorescence quantum yield from zero to values of about 0.60 is obtained for structurally related molecules of 1,3-diphenyl-2-pyrazoline such as 1-phenyl-3-mesityl-2-pyrazoline, 1,3-diphenyl-1,4,5,6-tetrahydro-1,2-diazine and benzaldehyde phenylhydrazone, all containing the same arylCNNaryl chromophore. The rigidity of the PMMA matrix slows down the rate of intramolecular motions such as ring inversion of rotational mobility within the fluorophore and lengthens the time interval between the electronic excitation and achievement of those conformations that give rise to fast non-radiative deactivation processes. In contrast with these compounds the fluorescence quantum yield of the nitrogen hetero aromatic compound 1,3-diphenylindazole decreases when going from MMA to PMMA at the expense of phosphorescence. The practical consequences of the effect of restricted intramolecular motions on the fluorescence quantum yield of fluorescent dyes in polymers are briefly discussed.