The pH dependence of the absorption spectra of bacteriorhodopsin and its photocycle intermediates was studied in the pH range 4.5-9. The spectra of the intermediates were determined from difference spectra taken during the photocycle with an optical multichannel analyzer. The data analysis was based on various criteria concerning the shape of the spectra, but no assumption was made about the kinetic model that describes the photocycle. The strategy for calculation of the spectra was one described earlier, as well as a newly introduced algorithm based on the Monte Carlo method. The search methods used gave very similar results. Like the absorption spectrum of bacteriorhodopsin in the above-mentioned pH range, the spectra of all the intermediates were found to be almost unchanged. The spectrum of intermediate M displayed a 2 nm, and that of intermediate L a 1 nm, red shift with rising pH, but this latter shift was within the overall error of the measurements. The similarity of all the intermediate spectra calculated with different procedures at all pH values points to the reliability of the method and the validity of the spectra. Averaging of the calculated spectra over the whole pH range furnished a well-determined set of intermediate spectra, suitable for further kinetic and spectroscopic studies of the photocycle.