A cyclic form of dihydroxyacetone phosphate, cDHAP, was obtained by the acidic hydrolysis of its dimethyl acetal (MeO)(2)cDHAP, and was crystallized in the hydrate, cDHAP-H (gem-diol; 1,3,2-dioxaphosphinane-2,5,5-triol 2-oxide) and the ketone, cDHAP-K (2-hydroxy-1,3,2-dioxaphosphinan-5-one 2-oxide) forms. The synthesis, MS and NMR analyses, crystallization and solid state structures determined by X-ray crystallography are described. The equilibrium state between the hydrate and ketone was established, revealing ∼86% of cDHAP-H in aqueous solution at room temperature. The striking structural feature of the ketone and hydrate forms of the same cyclic phosphate diester in the crystalline state is different conformations of the six-membered P/O/C/C/C/O 1,3,2-dioxaphosphorinane ring, namely chair (C) in cDHAP-H·H2O (9a-H) and skew (S) in cDHAP-K (9a-K). Apart from one previous report on the non-chair-puckered, disordered [(MeO)2cDHAP]- anion in the crystal of its ammonium salt (Ślepokura, K. Carbohydr. Res. 2008, 343, 113-131), this is the unique example of the cyclic phosphate diester in skew conformation. Both 9a-H and 9a-K crystallize with the phosphate group protonated, which has been so far rarely observed in the cyclic phosphates. Deformation of the phosphate group and flattening of the chair-puckered ring in 9a-H is typical of protonated P/O/C/C/C/O rings. However, the skew-puckered 9a-K shows different values of O-P-O angles versus the same lengths of P-O(endo) and P-O(exo) bonds.