An "internal heating effect" was discovered during the electrochemical etching (ECE) of some polymeric dosimeters such as polycarbonate (PC) and allyl diglycol carbonate (CR-39) employing a new multi-chamber ECE system capable of etching a large number of foils separately using small-volume chambers in parallel. The rate of heating is high enough to raise the etchant temperature significantly in a short period of time, e.g. from 25°C to 47°C in 20 min in CR-39. The rate of heating was smaller in polycarbonate. This heating effect was experimentally and theoretically investigated and discussed to be due to the "dielectric loss" in the polymer. The rate of heating is proportional to the polymer loss index (∊' tan δ), square of field strength, and frequency. Significant dimensional changes found in the foils, especially at high field strengths, were discussed to be due to the "electrostriction" phenomenon. In this paper, the results on the above findings and the effects of parameters are reported and discussed with a hope to partially understand some unknowns and to stimulate further research on the ECE mechanisms.