Crystals of M(AsO~3~OH) (M = Sr and Ba) were synthesized under hydrothermal conditions; single-crystal X-ray structure investigations were performed. The average structure of Sr(AsO~3~OH) exhibits three crystallographically different hydrogen bonds; two of them are related by -1 symmetry [space group P-1, a = 7.349(2) Å, b = 7.057(2) Å, c = 7.460(2) Å, α = 95.20(3)°, β = 104.78(3)°, γ = 88.11(3)°, Z = 4]. Weak superstructure reflections were detected which are responsible for a larger cell volume [space group P-1, a = 9.038(2) Å, b = 11.732(2) Å, c = 15.356(3) Å, α = 71.66(3)°, β = 75.16(3)°, γ = 89.11(3)°, Z = 16]. All inversion centres restricting the hydrogen bonds are avoided in the super cell. As a result, a completely ordered atomic arrangement is obtained. The arsenate tetrahedra are linked by hydrogen bonds to polar chains with branches in [110] and [110], respectively; they are linked by the Sr atoms. The compounds CaHPO~4~ (monetite), CaHAsO~4~ (weilite), α-SrHPO~4~
α-NaHSO~4~, and HgHPO~4~ have roughly the same arrangements of the M(= Ca, Sr, Na, Hg) and X(= P, As, S) atoms; however their hydrogen-bonding schemas are principally different. For the detailed comparison, the non-standard unit cell was chosen for Sr(AsO~3~OH) [space group F-1, a = 14.697(3), b = 28.230(5), c = 14.920(3) Å, α = 95.20(3)°, β = 104.78(3)°, γ = 88.11(3)°, Z = 64]. The structural refinement of Ba(AsO~3~OH) confirmed isotypy with γ-Sr(PO~3~OH), K(SO~3~OH), and K(SeO~3~OH) [space group Pbca
a = 8.581(2), b = 9.896(2), c = 19.417(4) Å, Z = 16]. Sr(AsO~3~OH) and Ba(AsO~3~OH) are characterized by short (strong) hydrogen bonds.