Hydrothermal treatment of quartz with 2 m K2CO3 solutions at 623 K and 1 kbar resulted in the formation of single crystals of the monoclinic polymorph of potassium hydrogen disilicate (KHSi2O5 or KSi2O4(OH)). Basic crystallographic data of this so-called phase I at room conditions are as follows: space group C2/m, a = 14.5895(10) Å, b = 8.2992(3) Å, c = 9.6866(7) Å, β = 122.756(10)°, V = 986.36(10) Å3, Z = 8. The structure was determined by direct methods and refined to a residual of R(|F|) = 0.0224 for 892 independent observed reflections with I > 2σ(I). The compound belongs to the group of chain silicates. It is based on crankshaft-like vierer double-chains running parallel to . The H atoms are associated with silanol groups. Hydrogen bonding between neighbouring double-chains results in the formation of ~5 Å wide slabs. The three crystallographically independent K cations with six to eight O ligands provide linkage (1) between the chains of a single slab or (2) between adjacent slabs. Structural investigations have been supplemented by micro-Raman spectroscopy. The interpretation of the spectroscopic data including the allocation of the bands to certain vibrational species has been aided by DFT calculations.