The effect of Lewis acids on spectroscopic properties and photodimerization of coumarin and 5,7-dimethoxycoumarin was investigated. Quantum yields of coumarin photodimerization increase in the presence of $BF_3{cdot}OEt_2$ but those of 5,7-dimethoxycoumarin decrease. The spectroscopic properties of the coumarin-$BF_3{cdot}OEt_2$ and 5,7-dimethoxycoumarin-$BF_3{cdot}OEt_2$ complexes were studied by UV, IR, $^1H$ NMR and fluorescence spectroscopy.