We have studied the spectroscopic properties of the B850 excitations of the peripheral light-harvesting complex (LH2) from Rhodopseudomonas acidophila utilizing single-molecule spectroscopic techniques. Employing highly parallel data acquisition schemes enabled us to obtain data with sufficient statistical significance to allow for detailed comparison with numerical simulations based on different types of disorder for the site energies of the B850 chromophores. From the analysis we come to the conclusion that the B850 pigments are subjected predominantly to diagonal rather than off-diagonal disorder. (C) 2004 Published by Elsevier B.V.