Abstract It has been shown by electron probe studies that the radiopaque dentin under restorations is the result of the presence of tin or zinc. In carious areas the reduced absorption of x-rays by decalcification may be overcompensated by the effect of tin or zinc present in this soft material. The concentrations found were for both zinc and tin approximately 1 to 10 per cent. Phantom experiments proved that the recorded concentrations can produce the observed increase in absorption of x-rays. Some extracted carious teeth which were filled and incubated for 2 to 3 years showed an increased radiopacity of the dentin similar to the radiopacity seen in radiographs of patients.