Prior experimental and theoretical studies have demonstrated that reactions involving H2CO and NH3 Can occur in water ice mixtures even at temperatures below 100 K. The key reaction between H2CO and NH3 generates monohydroxymethylamine, HOCH2NH2. The present study examines further reactions between this product and H2CO in ices. Quantum-chemical calculations at the MP2/6-31+G** level indicate that while H2CO may add across the OH bond in HOCH2NH2 to yield the amine-terminated polyoxymethylene species HOCH2OCH2NH2, it. is much more favorable to add across one of the two remaining NH bonds of the original ammonia molecule to yield (HOCH2)(2)NH. In fact, another H2CO may add across the final NH to form the tertiary amine (HOCH2)(3)N. On the basis of the favorable energetics of these reactions, it is suggested that one or both of these products. may be an, intermediate in the pathway to the formation of hexamethylenetetramine (C6H12N4), which is produced when ices containing methanol, ammonia, and water are subjected to UV irradiation and heated to room temperature.