Experiments at defined humidity, DSC measurements and X-ray powder patterns show that Cs2CdBr4 crystals are metastable at room temperature and dry air. They transform at enhanced humidity into a polycrystalline material composed from Cs7Cd3Br13 and CsCdBr3 microcrystals, which is stable up to about 480 K. Above this temperature, both components react forming Cs2CdBr4 polycrystalline solid.