The temperature dependences of the nuclear magnetic resonance (NMR) and the stress-strain hysteresis curves have been measured for K 3 H(SeO 4 ) 2 and Rb 3 H(SeO 4 ) 2 crystals grown using the slow evaporation method. By analysis of the 39 K and 87Rb NMR results, it has been confirmed that the phase below T C is ferroelastic with twin domains and that the phase above T C is paraelastic with a single domain. The ferroelastic domain switching that occurs in the two single crystals due to external stress was also studied. The critical stress values for converting the twin-domain state to the single-domain state were obtained. When external stresses of 0.6 and 0.4 MPa were applied to the K 3 H(SeO 4 ) 2 and Rb 3 H(SeO 4 ) 2 crystals, respectively, transitions from twin-domain to single-domain states were observed. The ferroelastic domain switching resulting from an external stress may be associated with the atomic rearrangements of the SeO 4 tetrahedra in K 3 H(SeO 4 ) 2 and Rb 3 H(SeO 4 ) 2 crystals; the ferroelastic domain switching is explained by the restriction of the rotation of the SeO 4 tetrahedra as a result of the breaking of all hydrogen bonds by the external stress.