We present the analysis of the photometric and spectroscopic properties of a sample of 29 low redshift (z < 0.6) QSOs for which both HST WFPC2 optical images and ultraviolet HST FOS spectra are available. For each object we measure the R band absolute magnitude of the host galaxy, the C IV (1550 Å) line width and the 1350 Å continuum luminosity. From these quantities we can estimate the black hole (BH) mass through the MBH - Lbulge relation for inactive galaxies, and from the virial method based on the kinematics of the regions emitting the broad lines. The comparison of the masses derived from the two methods yields information on the geometry of the gas emitting regions bound to the massive BH.