A resinous composition is made by: admixing (1) an organo-tin compound, having the structural formula: R.sub.1 R.sub.2 R.sub.3 SnX, where R.sub.1, R.sub.2 and R.sub.3 are organic groups, and X is a halide, hydroxide, acetate, butyrate, propionate, or dimethyl phosphate group, effective as a latent catalyst, (2) an amount of a reactive low viscosity epoxide diluent effective to dissolve the organo-tin compound without substantial heating, and (3) a cycloaliphatic epoxide or a glycidyl ester epoxide; wherein the weight ratio of cycloaliphatic epoxide or glycidyl ester epoxide: reactive epoxide diluent is between about 1:0.10 to 1:0.90, in the epoxide mixture; and about 0.00005 part to 0.0005 part by weight of organo-tin compound is used for each 1 part by weight of epoxide mixture.This resinous composition can be impregnated into a sheet material and cured to form insulation for an electrical member.