The effect of a electron temperature gradient on the I-Vb (electron current­ voltage) characteristic of the Langmuir probe is analysed. We find that [d(ln/I/edVb]-1 does not simply represent the local electron temperature when the temperature gradient is taken into account. [d(ln/I/edVb]-1 obtained from the probe measurements corresponds to an effective temperature, which is approximately the electron temperature a mean free path away from  the probe, the electron energy for calculating the mean free path being that of electrons  which are able to overcome the probe potential. A method of measuring the electron temperature profile using the probe is suggested.