In Escherichia coli, internalization and catabolism of organophosphonic acids are governed by the 14-cistron phnCDEFGHIJKLMNOP operon. The phnP gene product was previously shown to encode a phosphodiesterase with unusual specificity toward ribonucleoside 2`,3`-cyclic phosphates. Furthermore, phnP displays shared synteny with phnN across bacterial phn operons. Here the role of PhnP was examined by P-31 NMR spectrometry on the culture supernatants of E. coli phn mutants grown in the presence of alkylphosphonic acid or phosphite. The addition of any of these alkylphosphonic acids or phosphite resulted in the accumulation of alpha-D-ribosyl 1,2-cyclic phosphate and alpha-D-ribosyl 1-alkylphosphonate in a phnP mutant strain. Additionally, alpha-D-ribosyl 1-ethylphosphonate was observed to accumulate in a phnJ mutant strain when it was fed ethylphosphonic acid. Purified PhnP was shown to regiospecifically convert alpha-D-ribosyl 1,2-cyclic phosphate to alpha-D-ribosyl 1-phosphate. Radiolabeling studies revealed that 5-phospho-alpha-D-ribosyl 1,2-cyclic phosphate also accumulates in a phnP mutant. This compound was synthesized and shown to be regiospecifically converted by PhnP to alpha-D-ribosyl 1,5-bisphosphate. It is also shown that organophosphonate catabolism is dependent on the synthesis of S-phospho-alpha-D-ribosyl 1-diphosphate, suggesting that this phosphoribosyl donor is used to initiate the carbon-phosphorus (CP) lyase pathway. The results show that 5-phospho-alpha-D-ribosyl 1,2-cyclic phosphate is an intermediate of organophosphonic acid catabolism, and it is proposed that this compound derives from C-P bond cleavage of 5-phospho-alpha-D-ribosyl 1-alkylphosphonates by CP lyase.