Accurate measurement of temperature at the interface of the delivery electrode and the tissue during transcatheter delivery of radiofrequency energy (RFE) for ablation would provide better control of lesion production. Electromagnetic energy fields can affect the accuracy of temperature measurement with thermistors. An electrode probe was fabricated with a thermistor and an optical sensor in the center of the delivery electrode. Simultaneous temperature measurements during RFE delivery to cardiac tissue in the 37 degrees C bath showed good agreement between the sensors, indicating that the RFE field did not cause errors in thermistor temperature measurements with the electrode probe used. A second electrode probe was designed to determine optimal thermistor location. It was constructed using two thermistors with identical temperature-resistance curves. One thermistor protruded through a hole in the side of the delivery electrode and was thermally isolated from it. The other thermistor was bonded to the inner surface of the electrode with heat conductive epoxy. The electrode was placed in contact with cardiac tissue in a 37 degrees C bath of flowing saline with the protruding thermistor centered in the area to be heated. Temperatures measured at steady state during RFE delivery with the protruding thermistor were consistently higher than those of the inner wall thermistor, ranging from 1.8 degrees C difference at 46 degrees C to 8.3 degrees C difference at 75 degrees C interface temperature. The thermistor must be in contact with the tissue and thermally isolated from the delivery electrode for accurate determination of electrode/tissue interface temperature.