||Canonical Van Vleck perturbation theory (CVPT) is used to investigate rotation-vibration mixing of highly excited vibrational states of SO2 and H2CO. For SO2 we find a nearly complete separation of the rotational and vibrational degrees of freedom, even for J = 12 and E(vib) = 11 000 cm-1. In contrast, for H2CO we observe extensive mixing between rotational and vibrational degrees of freedom at similar rotational excitation but with E(vib) = 8000 cm-1. Although a-axis Coriolis coupling is pronounced, b- and c-axis Coriolis couplings play an important additional role in mixing states with different K(a) quantum numbers. The implementation of CVPT, the choice of internal coordinates, and the convergence of the results are discussed in detail.