Like the related cyanide Prussian Blue analogues, NCSperovskites are stable in the 'empty perovskite' ReO 3 structure. Indeed, there are only two reported NCS-perovskites contain- ing A-site cations: Cs{Cd(NCS) 3 } 24 and the double perovskite (NH 4 ) 2 {Ni[Cd(SCN) 6 ]} . Calculation of τ, using empirical cation sizes for molecular ions, suggests that organic A-site cations will likely be too large for A{M(NCS) 3 } perovskites [Fig. & ESI Section 3]. Indeed, for the well studied A + {Cd(NCS) 3 } composition, although changing A results in a wide variety of structures, including ferroelastics and non-linear optical materials, only Cs{Cd(NCS 3 )} adopts the perovskite structure. Introducing A-site vacancies allows us to reduce τ and thereby stabilise NCS-perovskites containing larger organic cations, increasing the range of available orderings.