From the data presented in the paper, the following points have been established: (1) Dihydro-umbelliferone and its derivatives, in which the pyrone double bond does not exist, do not give any fluorescence. (2) Similar to the carbonyl group cyano and phenyl groups enhance fluorescence emission markedly when present in the 3 position of umbelliferone. (3) No such effect is noticed when these groups occupy position 4. (4) A similar lack of influence is noticed in umbelliferone-3-acetic acid and its derivatives in which a methylene group breaks the conjugation. (5) Amongst monohydroxy coumarins, the presence of the hydroxy group in position 7 is most effective; in position 6, a feeble effect exists; in positions 5 and 8 no fluorescence is noticed. (6) Amongst dihydroxy-coumarins 6∶7, 5∶7, 8∶7, represent a decreasing series, the last giving no fluorescence. Derivatives of higher polyhydroxy-coumarins do not exhibit fluorescence. (7) Effect of methylation of the hydroxyl groups introduce marked changes in the capacity of the compounds to fluoresce and the colour of the fluorescence. 5-Methoxy-coumarin emits a feeble fluorescence whereas 8-methoxy compounds do not. (8) A few compounds exhibit fluorescence in the solid state. The above observations are explained from the effect of the structural features on the resonance of the molecule.