The crystal structures of potassium, ammonium, and rubidium tetrafluoborates have been redetermined. The structure of cesium tetrafluoborate has also been established. These 4 compounds are isomorphous and show orthorhombic symmetry, space groupPbnm. Atomic parameters have been obtained from a block diagonal least squares refinement using anisotropic temperature parameters. The final agreement residuals for observed reflections were KBF4,R = 0.072; NH4BF4,R = 0.075; RbBF4,R = 0.064; and CsBF4,R = 0.051.There are significant differences between some of the B—F bond distances in the [BF4]−ions. The shortest N—(H)F distance in NH4BF4is 2.914 Å (estimated standard deviation 0.005 Å) which is long for a hydrogen bond. The positions of the hydrogen atoms in the NH4+ion could not be established.