The physical properties of Al1−xCrxK(SO4)2·12H2O (x = 0, 0.07, and 0.2) single crystals as a function of temperature were studied by observing their 27Al nuclear magnetic resonance spectra and relaxation times. The effects of partial replacement of Al3+ ions with paramagnetic Cr3+ ions were examined on the basis of the physical properties of pure AlK(SO4)2·12H2O crystals. The phase transition temperatures TC varied depending on the concentration of paramagnetic ions. In addition, the spin–lattice relaxation time T1 of crystals containing Cr3+ ions was shorter than that of crystals without paramagnetic ions. The paramagnetic effects on T1 and TC were strongest for 27Al in Al0.8Cr0.2 K(SO4)2·12H2O crystals with 0.2 mol. % of Cr3+ ions.