The reaction between diisocyanates and diepoxides can produce polyoxazolidones. However, the reaction may be accompanied by concurrent cyclization of isocyanate to produce trimeric isocyanurate. Studies were performed with pure phenyl isocyanate or diphenylmethane-4,4′-diisocyanate (MDI) and the diglycidyl ether of bisphenol-A (DGEBA). Reactions were followed by FT-IR and differential scanning calorimetry in order to determine relative rates of conversion to polyoxazolidone and isocyanurate. Various catalysts were utilized, with the most effective being 2-ethyl-4-methylimidazole (EMI). Bulk reactions between MDI and DGEBA at 150°C resulted in considerable amounts of trimerization, thus giving crosslinked polymers. Solution reactions at 180–185°C in dry N-methylpyrrolidone using 0.1% EMI gave soluble and moldable polyoxazolidones.