Phase-lifetime spectrophotometry has been used to study the rate processes associated with intermediates in the photocycling pigments in membrane vesicles of mutant strains of Halobacterium halobium. Vesicles deficient in bacteriorhodopsin, but containing halorhodopsin, were monitored with light at 490 nm. Two relaxation processes, with kinetic parameters largely independent of pH over the range 6.2-7.8, were found to be associated with halorhodopsin photocycling in 4 M NaCl, 10 mM buffer at 23 degrees C. The average relaxation times are 0.94 and 11.4 ms. When vesicles deficient in both bacteriorhodopsin and halorhodopsin were monitored at 370 nm, a single relaxation process with an average relaxation time of 168 ms was detected. This process is independent of pH over the range 4.7-8.8. Examination of vesicles from ion flux mutants showed this slow process to be unrelated to halorhodopsin content and to derive from another photoreactive retinal pigment, possibly the recently described slow cycling pigment s-rhodopsin.