Coriolis coupling between free oscillation multiplets affects the determination of structure coefficients which linearly constrain the earth's aspherical structure. The coupling between fundamental spheroidal and toroidal modes may be modeled quite well by a complex frequency shift to the degenerate multiplet eigenvalues and a perturbation to the degree 2 axisymmetric structure coefficients. Observations of structure coefficients of coupled spheroidal modes clearly show the expected effect of coupling and we develop corrections to compensate for the phenomenon. Our results suggest that the P02 pattern of aspherical structure in the upper mantle may be biased by up to 50% if uncorrected structure coefficients are used in an inversion for earth structure. ¿ American Geophysical Union 1989