Five Ba(B'1/2B1/2'')O3 ceramic compounds (B' = Y3+, Gd3+, Nd3+, In3+; B'' = Ta and B' = Mg2+; B'' = W6+) have been prepared to investigate the origin of microwave losses. Infrared reflectivity and submillimetre transmission measurements as well as low-frequency dielectric measurements were performed and evaluated. An attempt was undertaken to divide the submillimetre losses into intrinsic lattice losses (multiphonon absorption) proportional to temperature and extrinsic temperature independent loss contributions.