Competitive protein binding radioimmunoassay (CPB-RIA) is a principal method for quantifying serum C Peptide concentration. The accuracy of this method is critically dependent on factors that influence the reaction between anti-C Peptide antibody (P) with 125I-C Peptide (M). We studied the influence of initial concentration of M, ionic strength, and viscosity on the reaction between M and P. A kinetic model for the the reaction between. Such model adjusts satisfactorily to the results. Bi-exponential and irreversible kinetics is determined. The results of the viscosity analysis show clear negative influence on the direct reaction rate. The ionic strength shows scarce influence on equilibrium and negligible influence upon the rate constant, which suggests that the variation resulting from the effect of the glycerol addition is not due to the influence of the dielectric constant of the solutions used. The effect of temperature shows activation parameters similar to the viscous flow energy of water, which suggests that the reaction is diffusion-controlled. The value of ΔHo for the immucomplex formation is positive, as is the case with endothermic processes.