The synthesis and single crystal structure of RbZrOAsO 4 are reported. This phase is the first fully characterised isostructure of potassium titanyl phosphate (KTP or KTiOPO 4 ) to contain only zirconium on the octahedral cation sites. The Zr1O 6 octahedron shows a displacement of Zr towards an octahedral edge, and the Zr2O 6 octahedron is virtually undistorted. The corresponding TiO 6 groups in titanium-containing KTP types both show a displacement of the Ti atom towards an octahedral vertex. Crystal data: RbZrOAsO 4 , M r =331.61, orthorhombic, Pna2 1 (No. 33), a=13.7538(6) A, b=6.8835(3) A, c=11.2712(5) A, V=1067.09(8) A 3 , Z=8, R(F)=0.034, wR(F 2 )=0.080.