Kinetic studies have been carried out of the monomer-dimer interaction of insulin, β-lactoglobulin, and α-chymotrypsin using stopped-flow and temperature-jump techniques. The pH indicators bromothymol blue, bromophenol blue, and phenol red were used to monitor pH changes associated with the monomer-dimer interaction. In all three cases a kinetic process was observed which could be attributed to a simple monomer-dimer equilibrium, and association (k1) and dissociation (k-1) rate constants were determined. The results obtained are as follows: for insulin at 23°C, pH 6.8, 0.125 M KNO3, k1= 1.14 × 108M-1s-1k-1= 1.48 × 104s-1; for β-lactoglobulin AB at 35 °C, pH 3.7, 0.025 M KNO3, k1= 4.7 × 104M-1s-1, k-1= 2.1 s-1; for α-chymotrypsin at 25 °C, pH 4.3, 0.05 M KNO3, k-1= 3.7 × 103M-1s-1, k-1= 0.68 s-1. The kinetic behavior of the separated β-lactoglobulin A and B was similar to that of the mixture. In the case of chymotrypsin, bromophenol blue was found to activate the enzyme catalyzed hydrolysis of p-nitrophenyl acetate, and a rate process was observed with the temperature jump which could be attributed to a conformational change of the indicator-protein complex. The association rate constant for dimer formation of insulin approaches the value expected for a diffusion-controlled process, while the values obtained for the other two proteins are below those expected for a diffusioncontrolled reaction unless unusually large steric and electrostatic effects are present.