The product of acetaldehyde and ammonia reaction, namely, 2,4,6-trimethyl-1,3,5-hexahydrotriazine trihydrate, was synthesized and identified using a combination of experimental (NMR spectroscopy, IR spectroscopy, melting point determination) and DFT-based theoretical approaches. A reaction mechanism was proposed. The reaction was shown to proceed via the formation of aminoalcohol, imine, and geminal diamine intermediates accompanied by cyclization of these species. The calculation results allowed us to build a potential energy surface of the acetaldehyde and ammonia interaction and determine the most energetically favorable pathway to yield acetaldehyde ammonia trimer. The reaction product was found in an energy minimum (-53.5 kcal/mol).