Time variations of the disk-averaged polarization of Venus at wavelengths of 270, 365, 550, and 935 nm have been analyzed in terms of average haze optical thickness, cloud-top pressure, and cloud absorption optical thickness at 270 nm (presumably due to gaseous SO2 absorption). The polarization data were deduced from observations made by the Pioneer Venus Orbiter between 1978 and 1990. Newton-Raphson iteration was used to derive the atmospheric parameter values from these observations. The required multiple scattering calculations were performed using the adding/doubling method. The atmospheric model consists of a cloud layer and a haze layer. The cloud and haze particles are composed of a concentrated sulfuric acid solution and have an effective radius of 1.05 μm and 0.25 μm, respectively. It was found that the disk-averaged haze optical thickness, cloud-top pressure, and cloud absorption optical thickness at 270 nm varied irregularly between 1978 and 1990. No trends were identified for the cloud-top pressure and cloud absorption optical thickness at 270 nm, but we did deduce a decrease in haze optical thickness during this 12 year period. Specific numbers are: (1) a haze optical thickness at 365 nm of about 0.25 in 1978 and less than 0.1 in 1990, (2) a cloud-top pressure that varies between 5 and 20 mbar, and (3) a cloud absorption optical thickness that varies between 0.0 and 4.0. Finally, our analysis revealed that the disk-averaged cloud-top pressure and haze optical thickness behaved anomalously in 1983. ¿ 1998 American Geophysical Union