Abstract The dielectric response of Ba 1− x Sr x TiO 3 ceramics and films ( x =0, 0.9, 1) was studied with a particular attention to the soft mode behaviour in the far IR and near-mm range and its comparison with the single crystal behaviour. To analyse the response quantitatively, we used commercial Fourier transform IR and monochromatic backward-wave-oscillator spectroscopy in both transmission and reflection modes and a rigorous evaluation. The soft-mode behaviour is extremely sensitive to the sample quality, particularly in films, and correlates with the lower low-frequency response. The most important factors for this behaviour in the polycrystalline films are grain boundaries and porosity and nano-cracks, which often appear in thicker polycrystalline films along some of the grain boundaries. The brick-wall model for grain boundaries and cracks discussed within the generalized effective medium approximation appears to be appropriate for describing the observed phenomena. In quasi-epitaxial films the macroscopic tensile stress, which influences the phase diagram very sensitively and induces ferroelectricity even in the pure SrTiO 3 , seems to play the most important role. From the soft-mode behaviour and its splitting, it appears that in the studied BaTiO 3 films all the phase transitions, as in single crystals, seem to be present, but smeared with phase coexistence regions down to 10 K. In the quasi-epitaxial BST-0.9 film a smeared ferroelectric transition appears near 150 K.