Pyridine-2,6-bis(monothiocarboxylic acid) (pdtc), a natural metal chelator produced by Pseudomonas stutzeri and Pseudomonas putida that promotes the degradation of carbon tetrachloride, was synthesized and studied by potentiometric and spectrophotometric techniques. The first two stepwise protonation constants (pK) for successive proton addition to pdtc were found to be 5.48 and 2.58. The third stepwise protonation constant was estimated to be 1.3. The stability (affinity) constants for iron(III), nickel(II), and cobalt(III) were determined by potentiometric or spectrophotometric titration. The results show that pdtc has strong affinity for Fe(III) and comparable affinities for various other metals. The stability constants (log K) are 33.93 for Co(pdtc)2(1-); 33.36 for Fe(pdtc)2(1-); and 33.28 for Ni(pdtc)2(2-). These protonation constants and high affinity constants show that over a physiological pH range the ferric pdtc complex has one of the highest effective stability constants for iron binding among known bacterial chelators.