Using a recently developed method, we show here that solvent effects on the first molecular hyperpolarizabilities of push-pull polyenes can be predicted by evaluation of the relaxation contribution beta(r) to the hyperpolarizability from vibrational infrared and Raman intensities and vibrational frequencies. The molecules studied in this work are a few organic polymethine dyes with different donor and acceptor groups. The analysis of their vibrational spectra confirms the key role played by the structural parameters of the polyene chain in determining the nonlinear optical response.