SAR measurements are realized by means of detected probes which have a correct behaviour in the case of CW emissions. For signals modulated by pulses raises a problem. The rise and the fall times of the pulse voltage detected are not the same. Moreover, the difference between these two times increases when the level of the electric field increases. This is the transient non-linearity (TNL) of the detected probes. This study evaluates the error induced by TNL in the case of SAR measurements of GSM mobile phones.