Transient linear dichroism and linear birefringence changes in the photocycle of bacteriorhodopsin at alkaline pH were measured with magnetically oriented purple membrane samples using the method of isotropic excitation (Borucki, B.; Otto, H.; Heyn, M. P. J. Phys. Chem. B 1999, 103, 6371-6383). At pH 10.4, the accumulation of the O intermediate is negligible, and N is the only intermediate that is relevant in the M decay and the recovery of the bR ground state. We introduced a new analytical approach that is model-independent and makes use of the transient linear dichroism in combination with isotropic absorbance changes. SVD analysis reveals that only one species (spectral component), namely, the N intermediate, is involved in the M decay at pH 10.4. The spectrum of this intermediate and its average anisotropy are determined from an eigenvalue equation. The transient linear dichroism data suggest a transition between two substates of the N intermediate with different anisotropies (i.e., with different orientations of the electronic transition dipole moment). Assuming that the polar angle of the transition dipole moment with respect to the membrane normal is 70degrees in the bR ground state, we get 66.3degrees (upper limit) for the first and 67.9degrees (lower limit) for the second N substate. The reorientation of the chromophore in the N-1 --> N-2 transition is probably associated with the movement of the protonated Schiff base away from Asp96, suggesting that it serves as a reprotonation switch for Asp96 by stabilizing the protonation state of the Schiff base. In contrast to measurements at neutral pH, the linear birefringence changes at alkaline pH contain a wavelength-independent component that is not due to the transient absorption changes and whose time dependence is correlated with the kinetics of the N intermediate. This result is attributed to an electro-optical effect caused by the large electric field generated by the transient negative charge on Asp96 in the N intermediate. Alternatively this change in Deltan may be due to the major conformational change in the cytoplasmic region of the membrane during N.