Objective To assess the role of pH in determining the solubility of Ca2+ in urine from normal subjects and stone formers and to determine the conditions which cause the formation of crystalline products. Materials and methods Urine, collected from normal subjects and urinary tract stone‐formers, was used in vitro. The concentration of ionized calcium, [Ca2+ ], and pH were measured with ion‐selective electrodes and the formation of crystalline products measured as the change in turbidity of the specimen. The composition of the crystalline products was analysed by infra‐red spectroscopy. Results The [Ca2+ ] decreased with increasing pH and showed a biphasic relationship with a critical pH (pHn ) at which Ca‐containing precipitates appeared. Compared with normal subjects, the mean (sd) value of pHn was significantly lower in urine from stone‐formers, at 7.67 (0.62) and 7.21 (0.54), respectively, and nearer to their voided pH of 5.92 (0.70) and 6.26 (0.71), respectively. The lower pHn in stone‐formers could not be explained by altered concentrations or binding properties of Ca2+‐binding ligands. pHn increased after diluting the urine and the value of pHn was strongly dependent on the [Ca2+ ] at a standardized pH. Conclusions Crystalline precipitates appear in urine at a critical pH which is closer to the voided pH in stone‐formers than in normal subjects and might explain the greater propensity of this group to form stones. The value of pHn is critically dependent on the urinary [Ca2+ ] and manoeuvres which reduced its concentration would reduce the tendency to form stones.