A novel noninvasive technique was developed to measure dissolution of the water-soluble component of a solid dosage form using indium 111 and perturbed angular correlation. The method involves time-delayed coincidence counting of two cascading gamma-rays that exhibit angular correlation. This angular correlation can be perturbed if the intermediate excited state of the nucleus is reoriented due to an interaction with its environment. When such an interaction occurs, as in a phase change (solid to liquid), the perturbation changes can be shown by anisotropy. A highly perturbed condition in the solid state results in low values (0.02-0.04), while increasing values of anisotropy indicate dissolution. Anisotropy values reach 0.14-0.15 when the total unperturbed physical state (liquid) exists. The worth of this technique was demonstrated by both in vitro and in vivo determinations of dissolution rates.