The existence of a minimum in the temperature dependence of the Rb87 spin-lattice relaxation time T1 in Rb1-x(NH4)xH2PO4 with x=0.35 indicates a tremendous progressive slowing down of the proton intrabond (O-H-O) jump motion from ~ 10-11 to ~ 10-7 sec not found in RbH2PO4 or other members of the KH2PO4 family exhibiting ferro- or antiferroelectric transitions. The occurrence of a linewidth broadening together with the T1 minimum exhibits a wide distribution of correlation times that develops near the onset of the low-frequency dielectric dispersion and loss characteristic of the glass state. The results can be described in terms of a cluster freeze-out model originally developed for spin-glasses. The NH4 reorientations freeze out much before the low-temperature glass state has been reached. © 1984 The American Physical Society.