The magnetic field dependence of the *H spin-lattice relaxation time in ammonium persulphate shows pronounced minima near the JH magnetic resonance frequencies of 1,200 and 2,200 kHz. These are interpreted in terms of a model involving cross-relaxation between *H in the N H 4 ion and 170 in natural abundance in the S2O g_ ions, the latter having a much shorter spin-lattice relaxation time. A theoretical analysis of the shape of the minima is used to derive values for the 170 quadrupole parameters. This analysis results in best estimate values for the quadrupole coupling constant of 6.75 ( ± 0.05) M Hz and an asymmetry of 0.30 ( ± 0.02). Such values are indicative of O H hydrogen bonding and suggest the S2O g_ ion is not undergoing rapid reorientation at temperatures below 320 K.