Temperature monitoring is a useful tool for radiofrequency ablation of accessory pathways. Impedance monitoring is also helpful, and an impedance fall of 20 ohm may predict coagulum formation. Therefore the purpose of this study was to prospectively quantitate the correlation between impedance and temperature during radiofrequency ablation. Thirty-three consecutive patients underwent radiofrequency ablation with a thermistor ablation catheter with continuous temperature and impedance monitoring during each energy application. The initial and final impedance and temperature measurements for 319 applications of radiofrequency energy were 109 ± 17 ohm and 102 ± 17 ohm (p < 0.0001), respectively, and 37 ± 1° C and 57° ± 11° C (p < 0.0001), respectively. Among the 319 applications of radiofrequency energy, 158 were associated with ≥20° C increase in tissue temperature, and 10 were associated with coagulum formation. A ≥20° C increase in tissue temperature was observed in 84 of 104 applications associated with a 5 to 10 ohm decrease in impedance (81% positive predictive value and 93% negative predictive value). The mean change in impedance for applications associated with and without coagulum formation was −19 ± 7 ohm and −6 ± 6 ohm (p < 0.0001), respectively, and coagulum formation never occurred with less than a 12 ohm decrease. In conclusions successful tissue heating without coagulum formation can be achieved by titrating the power to achieve a 5 to 10 ohm decrement in impedance. Impedance decrements beyond 10 ohm increase the likelihood of coagulum formation.