A series of new compounds MI(OCN) were prepared from the mixtures of MI2 and K(OCN) (M = Sr, Eu or Ba) by solid-state reactions that were controlled by differential scanning calorimetry (DSC) and differential thermal analysis (DTA) techniques. The presence of two phases is highlighted for BaI(OCN) which crystallizes in the Pnma (α-BaI(OCN)) and Cmcm (β-BaI(OCN)) space groups. The SrI(OCN), EuI(OCN) and β-BaI(OCN) compounds crystallize in the same orthorhombic Cmcm space group and the structure consists of a layered arrangement of [M2I2]2+ blocks separated by single layers of cyanate ions that are found to be related to the Sillén-type structure. The polymorphism of BaI(OCN) was also observed by analyzing the infrared (IR) spectra. This analysis showed for β-BaI(OCN) splitting bands, which were ascribed to the presence of more than one resonant cyanate form that can be induced by the dynamical disorder of OCN units. In contrast, the α-BaI(OCN) vibration modes suppose the existence of one cyanate form that adopts a defined orientation. The SrI(OCN) and BaI(OCN) compounds were doped with Eu2+. The luminescence spectra recorded under excitation at 340 and 350 nm revealed a blue emission of Eu2+ at 431 nm for the β-BaI(OCN) and SrI(OCN) phases and at 427 nm for the α-BaI(OCN) phase that was ascribed to the 4f65d1→8S7/2 (4f7) transition.