The reaction between carbonyldiimidazole (CDI) and copper (II) nitrate produces a new Cu (II) complex with nitrate as the counter anion. TGA, UV, and FTIR analyses confirmed that the coordination number of CDI in this complex is two. The acceleration effect of the complex in epoxy-dicyandiamide (DICY) curing systems has been evaluated by DSC and TMA, and the increasing viscosity of the mixture was monitored during the storage. The results revealed that the complex is not only very effective for the acceleration of epoxy-DICY systems, leading to a rapid gelation within 21 min at 120 degrees C, but it is also chemically stable at ambient temperature. This is reflected by the slow increase of viscosity of the accelerated curing systems stored at 35 degrees C, even over 56 days. In addition, the effects of the [Cu(CDI)(2)](2+) complex with different counter anions, that is, bromide, chloride, nitrate, sulfate, phthalate, and acetate, were compared using a series of tests. By comparing with N,N-dimethyl-N-phenylurea (fenuron), the widely used latent accelerator for DICY-epoxy curing systems, the complexes with bromide and nitride were found to be better, both regarding storage stability and for their acceleration effect. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3470-3476