AbstractPhoton‐correlation spectroscopy is a powerful technique for measuring the translational diffusion coefficient of particles and macromolecules in solution. In the study described here, this technique was used to analyze a specific dimerization process involving the association of two tRNA molecules through complementary anticodons. The tRNAs used in the analysis wereE. colitRNA 2Gluand yeast tRNAPhe. The experimental data on the concentration dependence of the observed diffusion constants are shown to agree well with theoretical predictions. From these data, the equilibrium constant of the association reaction was determined for dimers formed over a wide range of temperatures and in several different solution conditions. In solutions of 0.1Mionic strength at 22°C, the equilibrium constants vary from 1 × 105M−1in the absence of magnesium to 1.5 × 106M−1in 10 mMMg+2. The enthalpy and entropy changes for dimer formation in the absence and presence, 5 and 10 mM, of magnesium have been obtained from the temperature dependence of the equilibrium constant. The results show that both ΔHand ΔScontribute to the free energy of binding and that their relative contributions are similar for each solution conditi