Electron-nuclear double-resonance (ENDOR) spectra at 1.5 K of Cr5+(3d1) on tetrahedral As5+ sites in KH2AsO4 show lateral Slater protonic configurations. The dx2-y2 function couples with the two near protons. The reorientation of this unit in the four possible lateral configurations occurs in the paraelectric phase. The observed reorientation times are discussed. © 1977 The American Physical Society.