The recent incorporation of low-k dielectric materials (with k < SiO2 = 3.9) into multilayer Cu metallization, in order to lower the RC (resistance x capacitance) time constant, has heightened the search for a nondestructive, open volume (porosity), characterization tool. Positron annihilation spectroscopy (PAS) is one technique that can measure both the open volume pore size and pore distribution. In this paper beam-based positron annihilation lifetime spectroscopy (PALS) and Doppler Broadening (DB) are used to examine a number of low-k dielectric materials of particular interest to the semiconductor industry. In particular, insitu DB experiments were used to monitor changes in film porosity, as a function of annealing, for a series of 600 nm thick methyl-silsesquioxane-based films. Porosities were measured in capped and uncapped porous films, with up to 50% initial porogen loading, using a newly designed positron lifetime spectrometer on a magnetically guided beam. It will be shown that for porosities exceeding the percolation threshold Ps outdiffusion is seen through the red shift of the annihilation peak in the Doppler spectra and in lifetime measurements.