The pressure-dependencies up to 1100 MPa of the Q(y), Q(x), and Soret absorption bands for five- and six-coordinate monomeric complexes of bacteriochlorophyll a (Bchl a) in the solutions have been measured and quantitatively analysed. Here we show that the dependencies of the six-coordinate complexes can be accounted for within semiclassical theories, and the additional pressure-shifts in the dependencies of the five-coordinate complexes are caused by the Mg-O sixth axial ligand formation. Thereby the decrease of the Q(x) and Soret transitions energies, induced by the Mg-O bond, have been estimated. (c) 2007 Elsevier B.V. All rights reserved.