EPR of Cu'+, VO", SeO;, and other paramagnetic impurities and species were used to study the low-temperature phase transitions and twin-domain structure of (NHJ,H(SCO,)~ crystals. The major findings are (i) twinning in the monoclinic phases is of the merohedral type (there are three kinds of small domain related by rotations of C 120" abut the trigonal c axis), (ii) EPR of Cui+ or SeO; reveals the coexistence of the high-temperature and the lowtemperature phases below the trigonal R?m+monoclinic A2/o phase transition and on further transitions, however, into the two other low-temperature monoclinic phases a single phase was again observed and (iii) impurities and lattice point defects created by iunizingradiations were observed to influence the twinningandcrystalstructure of the host compound.