The crystal structure of Na1.55 K0.45Si2O5 has been solved and refined to a final residual R1 of 0.048 for 1208 independent reflections. The compound is monoclinic with space group P21/c (a = 4.845(1) Å, b = 8.647(2) Å, c = 11.992(3) Å, β = 90.31(4)°, V = 503.1(4) Å3, Mr = 189.40 u, Z = 4,λ(MoKα) = 0.71073 Å, Dx = 2.51 g/cm3, μ(MoKα) = 1.14 mm−1). The crystal showed twinning by pseudo-merohedry according to 2, a feature which was accounted for in the refinements. The compound belongs to the group of single-layer silicates. Individual sheets can be described as being built by the condensation of Zweier single chains of SiO4 tetrahedra parallel to the a axis or, alternatively, by condensation of Vierer single chains parallel to the b axis. The layers contain six-membered rings in UUDUUD or DDUDDU conformation. The stacking of the layers parallel to the c axis results in a three-dimensional structure in which the alkali cations reside between the layers for charge compensation. The distribution of the alkali atoms among the two crystallographically different M(1) and M(2) positions shows a definite preference by the larger potassium for the M(1) site. The smaller M(2) site is K-free.