A triple Langmuir probe was used for time-dependence measurements of electron temperature in the scrape-off layer of the CASTOR tokamak. The disturbing effect of poloidal electric fields on the probe signal was eliminated by subtracting the net disturbing signal obtained at the switched-off-probe bias voltage in another but identical tokamak shot. This difficulty can be avoided by using an array of four Langmuir probes. The fourth probe tip provides the compensating signal directly in one tokamak shot. (J.U.) 5 figs., 5 refs.