Sophisticated measurements were made on the nanosecond time-resolved absorbance change of the purple membrane of Halobacterium halobium under cw background light irradiation (440–800 nm, 11–441 mW/cm 2). A red-shifted transient species R 660 (K N, Q) was found in alkaline conditions (pH > 9.3). Background light intensity effect shows that (i) R 660 is photochemically formed from N 560 intermediate which is accumulated under background light irradiation because of the elongated lifetime in alkaline suspension, and that (ii) the slow decaying M 412 is not photochemically formed from N 560 but from bR.