Porous copolymer films were synthesized from a methylsilsequioxane: 1,2-bis(trimethoxysilyl)ethane (MSSQ: BTMSE) matrix and either an aromatic-core or aliphatic-core porogen at 10, 20, or 30 wt % porogen loading. Films were characterized using scanning electron microscopy (SEM), IR spectroscopy, and ellipsometry. Depth-sensing indentation experiments were performed to measure apparent film modulus, E-f', and hardness, H. Indentation load-displacement traces and SEM images were used to determine the threshold load for cracking, P-c. The aliphatic-core porogen produced a greater porosity film than the aromatic-core porogen for 10 wt % loading and smaller porosity films for 20 and 30 wt % loadings. IR spectra, normalized for film thickness and density, indicated decreased O-Si-O networking in porous MSSQ: BTMSE films. The combination of increased porosity and decreased O-Si-O networking led to a decreased apparent E-f' and H relative to the unmodified MSSQ: BTMSE film. However, low-porosity (approximately 7%), aliphatic-porogen MSSQ: BTMSE films are optimized relative to unmodified MSSQ with smaller dielectric constant and greater E-f' and H. (C) 2004 The Electrochemical Society.