The formation of carbamyl phosphate (CAP) in dilute solutions of cyanate (NCO-) and orthophosphate (Pi) was measured both in the absence and in the presence of a precipitated matrix of calcium phosphate (Pi.Ca). The second-order rate constant and the free energy of CAP synthesis were not modified by the presence of the solid matrix, indicating that synthesis occurs in the homogeneous Pi-containing solution. The elimination reaction of CAP to form NCO- and Pi followed first-order kinetics and the rate constant was the same whether or not calcium phosphate was present. Elimination was not complete, and the steady level of remaining CAP was that expected from the free energy of synthesis. The formation of pyrophosphate (PPi) was detected in CAP-containing medium only in the presence of calcium, showing a close correlation with the amount of precipitated Pi.Ca. Phosphorolysis of CAP followed a sigmoidal time course, compatible with adsorption of CAP to the solid matrix as a prelude to transphosphorylation. Addition of 5'-AMP and of short linear polyphosphates inhibited phosphorolysis of CAP. It is proposed that the presence of a solid phosphate matrix and the relative concentrations of cyano compounds, as well as those of nucleotides and inorganic polyphosphates, could have played a crucial role in the conservation of chemical energy of CAP and in its use in prebiotic phosphorylation reactions.