In this paper the Kalman-Filter algorithm is applied to the Arrhenius life model, normally used for thermal endurance characterization of insulating materials. It is shown how this algorithm can be used to derive estimates of Temperature Index and Halving Intervals, i. e. the indices recommended by IEC216, using thermal endurance data relevant to three insulating materials or systems. Depending on scattering of the experimental points from the assumed linear model, especially of the points at the lowest temperatures, estimates performed by Kalman filter and according to IEC216 may considerably differ. Tests of the validity of the model, which are available in the Kalman procedure, show that there is a high probability that the linear model is not valid at the lowest test temperatures.