In this paper the influence of the sulphur containing amino acids methionin and cystein on the fluorescence behaviour of riboflavin in aqueous solution is studied. Measurements are carried out mainly at pH=5, 7, and 9. The results are compared with the fluorescence behaviour of riboflavin in pure aqueous solution, methanol (CH3OH), and in dimethylsulphoxide (DMSO, (CH3)2SO)