Studies of phosphate retention by four Kenyan soils showed dependency on pH. Precipitation by Al and Fe in the soils accounted for maximum retention at pH values' 2-3.76. Calcium precipitates were formed in the pH range 6.72-10.42. Retention curves were consistent with an adsorption mechanism. Retention of binary mixtures of phosphate-molybdate and phosphate-oxalate systems was quantified as a function of pH. The molybdate ion significantly inhibited phosphate retention over the entire pH range, reflecting preferential adsorption of molybdate ions to phosphate by the soils. The oxalate ion altered the retention of P by freeing Al and Fe ions held in solution in precipitation form. The ability of the constant capacitance model to quantify the adsorption of phosphate was evaluated using model constants. The model generally described the shape of adsorption curves of P retention against pH but did not quantitatively account for the amounts of P retained by these soils.