The synthesis of allantoin from glyoxylic acid and urea has been studied using several heterogeneous catalysts. Organic resins and nanocomposites, particularly sulfonic resins, lead to higher yields than zeolites. Polymer morphology has a significant influence on the performance of the catalyst. The possibility of using the same type of solid acid in the reaction of allantoin with phenol has also been explored. However, this route requires a larger amount of acid and leads to lower para/ortho selectivity than the direct route from glyoxylic acid and urea.