The kinetics of a diffusion‐limited pairing reaction between oppositely charged impurity ions in a solid have been studied by observing the capture of mobile Li+ions by B−ions in Si. The kinetics were determined by measuring resistivity vs time after the method of Reiss, Fuller, and Morin [Bell System Tech J.35, 535 (1956)]; as pairing proceeds, the resistivity decreases because of the disappearance of the charged impurity scattering associated with unpaired ions. Measurements were made between 2° and 35°C. The observed kinetics are not of first order, and are best described by a model in which pairing is largely a random process with little correlation between particular Li+and B−ions. Diffusion constants of Li+calculated from the kinetics are in accord with previous ion‐drift results.