Four kinds of epoxy resins: cresol novolac, tris-hydroxyphenylmethane, tetramethylbiphenol, and bisphenol A, were cured with phenol novolac epoxy resins. Characteristics of these epoxy compounds were studied by the positron annihilation lifetime (PAL) technique. Glass transition temperatures, thermal expansion coefficients, and volume of intermolecular-space holes among polymer chains were obtained from the lifetime, tau3, of the long-lived component of ortho-positronium. It was revealed that, at the glass transition temperature, T(g), the volume of the hole created among polymer chains expanded 1.4 times the volume at room temperature. The smaller flexural modulus of tris-hydroxyphenylmethane than that of the other samples was explained by the volume of intermolecular-space holes obtained from tau3. Aging effects were seen in the data of the intensities, I3, of ortho-positronium, which became smaller after heating the samples above T(g). I3 and tau3 were strongly affected by the density of cross-linkings and their chemical structures. The larger the density of cross-linkings, the smaller I3 and higher T(g) were obtained. Epoxy compounds with the higher water-absorption rates had larger intermolecular-space holes. (C) 1993 John Wiley & Sons, Inc.