To investigate the cause of the d.c. charged conductivity state of kidney stones, the effect of annealing and sample thickness on thermally stimulated polarization was studied. It was observed that the peak current decreased and the peak temperature rose with increasing annealing temperature and the sample thickness. The sharpness of the single peak decreased and the activation energy increased with both parameters. The results have been explained on the basis of the loss of water attached to calcium oxalate and calcium phosphate, denaturation of collagen and the ionic nature of the main constituents of the kidney stone.