This work investigated suggested interactions between the phosphate rock component and the acidic solution formed by hydrolysis of monocalcium phosphate (MCP) as partially acidulated phosphates absorb water. Two approaches are reported. First, various materials were incubated in humid atmospheres for up to seven weeks and solubilities determined. Secondly, rock particles were mixed with a synthetic solution based on the hydrolysis products of MCP. Neither approach gave any enhanced solubility in neutral ammonium citrate solution which showed that the residual phosphate rock had not reacted.