In the absence of oxygen, NTMP (nitrilotrismethylenephosphonic acid) is oxidized by aqueous suspensions of the Mn-III-containing mineral manganite (MnOOH), yielding iminodimethylenephosphonic acid (IDMP) as the sole phosphonate-containing product, along with orthophosphate and two equivalents of Mull. Hence, both C-P and C-N bond cleavage take place. The reaction proceeds via formation of a Mn-III-NTMP surface complex and oxidation of NTMP by Mn-III via a carbon-centered methylene radical. Mn-II strongly inhibits the reaction, presumably by competition for available surface sites. Homogeneous NTMP oxidation in Mn-II and O-2-containing solutions yields IDMP, formate, phosphate, and hydrogen peroxide. Interception of the methylene radical by O-2 leads to N-formyl-IDMP and a second not-identified product. The dual presence of MnOOH and O-2 yields autocatalytic NTMP oxidation, owing to the generation of dissolved Mn-II. Although the ultimate oxidants in the Mn-II-NTMP-O-2 and MnOOH-NTMP systems are different (O-2 versus Mn-III), both oxidation reactions begin with C-N bond cleavage. This finding supports the hypothesis that Mn-III is generated in the Mn-II-NTMP-O-2 system.