The kinetics of single‐interface motion in the diffusionless transformation of Au‐Cd single crystals containing 47.5 atomic percent Cd were studied. A diffusion‐controlled relaxation phenomenon was found with an activation energy of the order of 24,000 calories per mole. The two observed characteristics of diffusionless transformation of this alloy, that the transformation does not take place isothermally, and that the velocity of transformation on cooling is considerably and consistently larger than that on heating, can be successfully interpreted from the relaxation hypothesis.