The 9.4 GHz electron paramagnetic resonance (epr) of Gd3+ impurity ions in NH4Sm(SO4)2∙4H2O single crystals has been studied at 101–297 K. The 294 K epr spectra show that the Gd3+ impurity ion substitutes for the Sm3+ host ion in two magnetically nonequivalent complexes corresponding to the tetramolecular unit cell. These spectra were analyzed using an orthorhombic symmetry spin-Hamiltonian. The observation of resolved Gd3+ impurity ion spectra in the presence of paramagnetic Sm3+ ions has been interpreted in terms of a random modulation of the interaction between the Gd3+ impurity and Sm3+ host ions by rapid spin–lattice relaxation of the Sm3+ ions. The spin–lattice relaxation time of the Sm3+ ions has been estimated at 294 K from the impurity ion epr linewidths and is found to be consistent with an Orbach resonance process. The value for τ1, has also been computed for Nd3+ ions in NH4Nd(SO4)2∙4H2O from the epr linewidth data in the literature. It has been shown for the first time that NH4Sm(SO4)2∙4H2O single crystals have the phase transition sequence[Formula: see text]