Laboratory incubation studies designed to measure chloride (Cl-), soil solution osmotic potential, and soil pH effects on nitrification of added NH+4 were conducted in order to help explain effects of Cl-containing fertilizers and liming on soil NH+4 and NO-3 concentrations measured in field experiments. Samples of an Aquultic Argixerolls (pH 5.5, 6.0, 6.4, and 6.7) and a Xeric Haplohumults (pH 4.9, 5.5, and 6.2) were amended with 100 mg NH+4-N kg−1, maintained at −10-kPa matric potential and incubated at 25°C. Ammonium was added as either NH4Cl or (NH4)2SO4 salt solutions with four osmotic potential levels ranging from −96 to −692 and −90 to −669 kPa, respectively. Osmotic potential gradients were obtained by adding KCl to the NH4Cl solutions and K2SO4 to the (NH4)2SO4 solutions. At 0, 3, 9, 14, and 28 d, samples were extracted with 2 M KCl and analyzed for NH+4, NO-3, and NO-2. At 15 d, soil solutions were recovered by centrifugation and analyzed for Al, Ca, Mg, K, Na, Mn, NH4, NO3, PO4, SO4, Cl, pH, and osmotic potential. Soil solution osmotic potential was lower in soils amended with Cl- than with SO2-4 salt solutions. In both soils, nitrification was inhibited by Cl- and by decreasing soil solution osmotic potential. Chloride inhibition of nitrification was greatest (⋍30%) at low soil pH (4.9-5.5) but disappeared or decreased markedly in magnitude above pH 6.0 to 6.2. Because of soil solution osmotic potential and pH by Cl- interaction effects, inhibition of nitrification would be expected to be greatest where Cl-containing fertilizers are applied in a band on moderately acid (pH 5.0-5.5) soils.