Abstract The effects of adding Co or Cu ions as paramagnetic impurities to [N(CH 3) 4] 2ZnCl 4 single crystals were investigated. Specifically, we measured the temperature dependences of the 1H relaxation times for the mixed crystals [N(CH 3) 4] 2Zn 1− x Co x Cl 4 ( x = 0 , 0.1, and 1) and [N(CH 3) 4] 2Zn 1− x Cu x Cl 4 ( x = 0 , 0.1, and 1). We found that the addition of these impurities did not affect the phase transition temperatures and the lattice constants of the mixed crystals, but did cause a change in the activation energy, E a , obtained from the spin–lattice relaxation time, t 1 . Thus the presence of Co or Cu impurities affects the potential barrier of the molecular motion of 1H nuclei in the CH 3 groups. The changes in activation energy caused by introducing Co or Cu impurities into the mixed crystals may stem from the Co and Cu ions being smaller than the Zn ions they replace, and also from differences in the electron structures of the metal ions, in particular the structure of the d electrons, which screen the nuclear charge from the motion of the outer electrons. Collectively, our results indicate that introducing Co or Cu impurities into [N(CH 3) 4] 2ZnCl 4 single crystals mainly affects the molecular motions at high temperature, and that the molecular motions and activation energy are dependent on whether the paramagnetic impurity is Co or Cu.