Nonlinear optical properties of poly(vinyl alcohol) (PVA)/polyaniline (PAni) blends were measured with the single-beam Z-scan technique with Fourier analysis. The results obtained with continuous wave (cw) excitation indicated that the self-phase modulation had a thermal origin. Besides the Z-scan technique, we also employed the time-resolved mode-mismatched thermal lens (TL) technique to obtain the temperature coefficient of the optical path length, ds/dT, and the thermal diffusivity coefficient, D, for the specific concentrations used in our blends. ds/dT varied between -0.8 and - 1.0 x 10(-4) K-1, whereas the thermal diffusivity varied between 1.0 and 1.3 x 10(-3) cm(2)/S. The TL technique was further used to study the aging of the blends as they were heated to 90 degreesC. Unlike the electrical conductivity of PAni films, which presented a strong dependence on the doping level, the thermooptic properties presented only a slight variation with doping. This feature indicated that the PVA/glutaraldehyde network made the main contribution to the thermooptic properties (D and ds/dT in the PAni blends. Similarly, dimethyl sulfoxide as a solvent determined the thermooptic properties of PAni solutions.