The photo-induced transformation of methyl and methyl acetic acid guanidine derivatives, which are the decomposition products of several pollutants, was studied adopting titanium dioxide as photocatalyst. The in-depth investigation of the overall process involved studying the evolution of intermediate compounds, the fate of organic nitrogen and the rate (and extent) of carbon mineralization. Intermediate products were identified with an HPLC/tandem MS instrument, thus enabling a transformation mechanism applicable to methyl-guanidinic derivatives to be defined. The derivatives are chiefly transformed through a combination of reductive and oxidative processes. Photocatalytically assisted hydrolysis, through sequential reactions involving (OH)-O-center dot/e(-)/H+, induces a demethylation process that, in all cases, leads to the formation of guanidine, then (very slowly) to its mineralization. Similarly, also with creatine and creatinine the main transformation pathway proceeds through a photocatalytically assisted hydrolysis that leads to the formation of 2-imino-imidazole-4-one (from creatinine) and guanidine acetic acid and methyl guanidine (from creatine). Guanidine is the ultimate intermediate compound recognized in these cases, too. The fate of nitrogen was also evaluated in all cases. The organic nitrogen is mainly released as nitrate ions, with [NH4+]:[NO3-] ratios ranging from 1:2 for creatine to 1:8 with methylguanidine. (c) 2005 Elsevier B.V. All rights reserved.