We measured positron annihilation lifetime spectra in mesoporous low dielectric constant (low-k) methyl-silsesquioxane (MSSQ) films versus porogen load Φ from Φ=0% to 50%. The ortho-positronium lifetime parameters were obtained using both the maximum entropy and discrete lifetime analyses. Open and closed porosity distributions and the average radius of closed pores were obtained. The total porosity and the fraction of open/closed porosities were evaluated. The total porosity increases linearly with porogen load, consistent with the porosity obtained from density measurements. Open porosity occurs from 20% porogen load upwards. © 2002 American Institute of Physics.