An ideal porous methyl-silsesquioxane (MSQ) film for low-k dielectrics would consist of a network of small, closed pores with homogeneous size distribution. In this work, we studied the pore size and interconnectivity between pores for porous MSQ films templated with various types of triblock copolymers (pore generator) using positrons by measuring ortho-positronium (o-Ps) 3gamma emission, Doppler broadening of annihilation photons (DBAP), and o-Ps lifetimes. Results show that the depth profiles of the pore structure depend on the molecular mass of pore generators. DBAP measurements show an interfacial layer between the porous films and the Si substrate, which is believed to be the native oxide on the Si wafer surface. Our results suggest the counter intuitive interpretation that porous MSQ films templated by triblock copolymers with larger molecular-mass have smaller and more closed pores, while those with smaller molecular mass and lower ethylene oxide fractions tend to contain larger and more-interconnected pores. (C) 2002 Elsevier Science B.V. All rights reserved.