Hydrostatic pressure effects up to 907 MPa on absorption spectra of chlorophyll a molecules dissolved in diethyl ether have been studied at ambient temperature both experimentally and theoretically using quantum chemistry methods. The fluorescence spectra are studied only experimentally. The data suggest that coordination interactions between the central Mg atom of the chlorophyll and solvent molecules along with interactions that modify the porphyrin skeleton of the solute are responsible for the observed differences of pressure dependence of the Q(y), Q(x), and Soret spectral bands. The coordination number of the Mg atom changes from five to six between 400 and 600 MPa. (C) 2004 Elsevier B.V. All rights reserved.