The formation of formaldehyde from phenylacetaldehyde and phenylalanine, and the contribution of both formaldehyde and ammonia to the production of PhIP from phenylacetaldehyde and creatinine were studied in an attempt to clarify the reaction pathways that produce PhIP. Formaldehyde was produced by thermal degradation of phenylacetaldehyde and, to a lesser extent, also by degradation of phenylalanine, phenylethylamine, styrene, and creatinine. When formaldehyde was added to a mixture of phenylacetaldehyde and creatinine, PhIP yield was multiplied by nineteen. When formaldehyde and ammonia were simultaneously present, PhIP yield was multiplied by fifty and the Ea of the reaction decreased by 61%. All these results point to formaldehyde and ammonia as the two additional reactants required for PhIP formation from both phenylacetaldehyde/creati(ni)ne and phenylalanine/creati(ni)ne mixtures. A general pathway for PhIP formation is proposed. This pathway is suggested to be the main route for PhIP formation in foods.