There is considerable clinical evidence that the oral administration of potassium citrate significantly reduces the incidence of calcium oxalate stone formation in the urinary tract. The effectiveness of citrate ions in preventing stone formation could be due to the reduction in the concentrations of calcium and oxalate ions caused by complex ion formation with the citrate ions and/or due to the inhibition of the crystallisation of calcium oxalate. This paper reports an experimental study aimed at elucidating the role of citrate complexes in preventing urolithiasis. An experimental method is described which allows the identification of two hitherto unknown complexes CaOx cit3- and (Ca cit2)4-. The stability constants of these complexes have been determined, respectively, as log K = 4.54 +/- 0.08 and beta 2 cit = 5.15 +/- 0.14 (25 degrees C, I = 0.16). The inclusion of these complexes in ion-equilibrium calculations led to the conclusion that the effectiveness of the citrate ion in preventing calcium oxalate stone formation is due to its inhibition of agglomeration or growth of calcium oxalate crystals rather than any significant reduction in the degree of supersaturation of urine.