Heat-curable epoxy resin systems comprising (a) at least one epoxy resin, (b) at least one polycarboxylic acid anhydride, and (c) 1,4-diazabicyclo(2,2,2)octane or a C.sub.1 -C.sub.4 alkyl-substituted derivative of 1,4-diazabicyclo(2,2,2)octane as curing accelerator, and wherein one portion of the curing accelerator (c) is replaced by (d) a complex of a boron halide with an amine, have good reactivity at elevated temperatures and comparatively high stability at temperatures below c. 80.degree. C.