The formation of H aggregates and structural transformations of an amphiphilic 7-aminocoumarin dye derivative monolayer have been studied at the air-water interface. In situ UV-visible surface reflection spectroscopy, elasticity and surface potential measurements were used to characterize the floating monolayers. There was no large difference between the absorption spectrum of the chloroform solution of the dye and the surface reflection spectrum measured at the air-water interface before the compression. A blue shift of the absorption maximum was observed during the monolayer compression. The elastic modulus as a function of surface pressure exhibited some peculiarities and revealed the existence of several phases and phase transitions during the compression. The elasticity data were consistent with the spectroscopic results.