Near-infrared spectra from the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on Venus Express have been used to investigate the vertical structure and global distribution of cloud properties on Venus. The spectral range covered by VIRTIS is sensitive on the nightside to absorption by the lower and middle cloud layers, which are back-lit by radiation from the lower atmosphere and surface. The cloud model used to interpret the spectra is based on previous work by Pollack et al. (1993) and others, and assumes a composition for the cloud particles of sulfuric acid and water, with acid concentration as a free parameter to be determined. Other retrieved parameters are the average size of the particles and the altitude of the cloud base in the model. The sensitivity to these variables across the measured spectral range (1.5 to 2.6 μm) is investigated, and radiances at suitable pairs of wavelengths are used in model branch plots to recover each variable independently. Model spectra are generated using the NEMESIS radiative transfer and retrieval code (Irwin et al. 2008).