γ‐irradiation of MoO 43− doped KDP crystals is shown to produce the paramagnetic centre MoO 43−. The g‐values of the MoO 43− centre suggest that its ground state is d z 2. This radical is used as a probe to detect the ferroelectric phase transition of the KDP crystal. The EPR spectrum of MoO 43− in the paraelectric phase shows a quintet proton hyperfine structure which changes to a triplet as the temperature is lowered below a critical temperature of 144 K. Copyright © 1987 WILEY‐VCH Verlag GmbH & Co. KGaA