p-chloronitrobenzene (PCNB) and p-chlorobromobenzene (PCBB) crystallize in the centrosymmetric space group P2(1)/c with two molecules per unit cell. The space lattice will have an equal number of points with molecules facing in opposite directions. As a consequence, these compounds exhibit an orientational rigid disorder. In this work, we have measured the temperature dependence of the chlorine nuclear quadrupole spin-lattice relaxation time (T-1), linewidth, and resonance frequency for both compounds for temperatures higher than 80 K. Both compounds exhibit an inhomogeneously broadened line shape and a "normal" Bayer-type temperature dependence of the resonance frequency. The analysis focuses on the identification of the dominant relaxation process at high temperatures (T>240 K in PCNB and T>260 K in PCBB). It is shown that T-1(T) reflects the existence of 180 degrees molecular reorientations through a modulation of the crystalline contribution to the electric field gradient.