Raw skim milk was diluted 1000-fold using distilled water or various salt solutions as specified. Smooth, hyperbolic profiles of coagulation rate v. pH for casein were calculated from recordings of turbidity (400 nm) with time. The effects of pH, cation type, anion type and cleavage of k−casein by chymosin (EC 184.108.40.206) were determined. The maximum of pH-coagulation rate profiles decreased by 63, 85 and 94% when the skim milk diluent was changed from water to salt solutions of NaCl (100 mM), CaCl2 (50 mM) or MgCl2 (50 mM). The maximum of the pH–coagulation rate profile was 15 times greater when the Ca salt was changed from CaCl2 to Ca(SCN)2 (50 mM). The highest pH at which casein coagulation occurred increased from 4·45 to > 6·0 when Cu2+ (1 mM) was included with casein micelles dispersed in CaCl2 solution (50 mM). The addition of chymosin to casein micelles suspended in CaCl2 solution (70 mM) eliminated the inhibition of casein coagulation by Ca2+ at pH 4·5. It is proposed that ions such as Mg2+, Ca2+, and Na+, which generally associate with casein phosphate and carboxylate groups, increased the H+ concentration required to initiate the coagulation of casein, because H+ must displace bound Ca2+, Mg2+ or Na+ to reduce repulsive hydration forces between casein micelles, allowing attractive hydration forces (e.g. hydrophobic phenomena) to cause casein coagulation. Furthermore, it is proposed that ions such as Cl−, Br−, and SCN− bind to lysine, arginine and histidine groups and thereby decrease repulsive hydration forces between cationic casein micelles.