The thermal dehydration of hexathiocyanatobismuthates(III) of lanthanides of the general formula Ln[Bi(SCN)6] . nH2O, where Ln is La, Ce, Pr, Nd, Sm, Gd, Dy, Er, Y and n is 3-5, has been examined. The thermal analysis curves of the complexes and the results of chemical and X-ray analyses of the products are presented. From these results, the mechanism of the thermal dehydration was established. The reaction order and activation energy were calculated by the Coats-Redfern and Zsako methods.