The high-resolution FTIR spectra of the ν5 band of monoisotopic D3 70GeH have been fitted by two sets of parameters corresponding to different reductions of the effective rotational Hamiltonian for the doubly degenerate vibrational state of a molecule of C3ν symmetry. The unitary equivalence of both sets of parameters has been verified by using relations which follow from the theory of the reduction of the effective rotational Hamiltonians of C3ν molecules. This made it possible to determine "true" values of the parameters of the Δl = ±2, Δk = ∓1 and Δl = 0, Δk = ±3 interactions, as well as of the higher-order terms ηJ and ηK in the z-Coriolis coupling. Relations between unitary equivalent sets of spectroscopical parameters are compared for the ν10) = 1 vibrational state of CF3CCH, ν6 = 1 of CDF3, and ν5) = 1 of D3 70GeH.