UV-Vis-NIR diffuse reflectance spectra of heteropoly acids of the type of H4PVMo11O40.xH2O were recorded with improved spectroscopic equipment for different times on helium stream at room temperature. A broad asymmetric absorption band with a maximum at 740-780 nm in the visible range and a shoulder at 900-1000 nm in the near infrared range is identified. With increasing time on stream the intensity of this band grows appreciably: simultaneously, the water peaks at 1430 and 1925 nm decrease. Focusing on a single mixed-valence V4+-Mo6+-cluster, a model is suggested for the description of the experimental results. The model takes into account electron transfer, low symmetry crystal fields and vibronic interaction. The absorption band shape calculation is performed within the semiclassical limits. The band shape is shown to be determined both by the optical d-d transitions in V4+ and Mo5- and the intervalence V4+-Mo6+ optical transition. Appreciable changes in the band intensity are attributed to the increase in the electron transfer parameter due to water loss. Quite good agreement with the experiment was found for a reasonable set of transfer and vibronic parameters.