AbstractMeasurements of the rate of growth R of a potassium aluminium alum crystal and a sodium chlorate crystal are presented. For alum, when the relative supersaturation σ lies between 3 × 10−5 and 10−2, R is found to be proportional to σ but for σ greater than 10−2 when spontaneous nucleation occurs, some points (showing considerable scatter) are obtained which lie above the linear curve. For sodium chlorate in the range 3 × 10−5 ⪅ σ ⪅ 5 × 10−4 a distinctly non‐linear (nearly parabolic) (R, σ) law is observed and for the range 5 × 10−4 ⪅ σ ⪅ 1.5 × 10−3 a linear curve is found which upon extrapolation to low values of σ does not pass through the origin. The theoretical (R, σ) curve resulting from the surface diffusion model of Burton, Cabrera, and Frank [1, 2] fits the measured (R, σ) points of sodium chlorate in a satisfactory way. It follows from the experiments that R does not depend on the velocity of flow of the solution.