The radial ion temperature distribution in a 2600 amp hydrogen arc in a 30 kG axial magnetic field was measured spectroscopically with a Fabry-Perot interferometer by evaluating the half width of Doppler broadened carbon lines. As the admixture of methane had to be low, in order to prevent a strong reduction of the temperature on the axis, the intensities of the C III and C IV lines were relatively weak. Consequently an interference filter with narrow transmission band width was used instead of a spectrograph in front of the F.-P.-interferometer. The broadening of the spectral lines, which were used for measurement, was caused mainly by the thermal Doppler effect. The rotation of the plasma and the macroscopical Doppler effect resulting therefrom did not disturb the measurements. The ion temperature, which was found to be 1.4 × 10 5 °K on the axis, was determined from the half width of the profiles of a C III spectral line. Since the discharge was observed side-on, the measured integrated values were reduced to radial profiles by using appropriate inversion formulae.