Various substituted 4-methylcoumarin derivatives were synthesized in order to obtain photostable derivatives with UVA absorption property. It was found that substitution positions affected maximum absorption wavelength, whereas types of substituents, whether hydroxy or alkoxy groups, caused no significant effect. Photostability, however, was affected by both the substitution positions and the types of substituents. An acid-catalyzed mechanism through enolization coupled with pre-organization via hydrogen bonding between two coumarin moieties is proposed as an explanation for the different extents of the [2+2] cycloaddition (dimerization) amongst different derivatives, which results in a different photostability amongst them. Photostable coumarin derivatives with an absorption maximum in the UVA region are reported.