An epoxy oligomerization catalyst can be employed to prepare an epoxy resin employing a method including admixing a first epoxy resin having a first epoxy equivalent weight of from about 100 to about 600 with a diphenolic compound, and a catalyst, thereby forming a second epoxy resin having a second epoxy equivalent weight of from about 200 to about 10,000; wherein the catalyst is a guanidinium catalyst. The second equivalent weight is greater than the first equivalent weight.