The dielectric properties, infrared (IR) dielectric response and Raman spectra of tunable microwave (MW) Ba 1Àx Sr x TiO 3 (x = 0.4, 0.5, 0.6 and 0.7) ceramics were studied systematically as functions of composition and temperature. It was found that, with increasing the content of Sr from 40% to 70%, the permittivity of the Ba 1Àx Sr x TiO 3 ceramics decreased from 7200 to 640 while the dielectric tunability decreased from 50.1% to 5.4% measured at 10 kHz. It is particularly interesting that the MW dielectric loss tangent was significantly decreased from 1.5 Â 10 À2 to 7.4 Â 10 À4 . Complex permittivity spectra obtained by fitting the infrared data were also extrapolated to MW frequency and compared with the dielectric data near 1 GHz. For the samples with x = 0.4 and x = 0.5, the dielectric loss measured at $1 GHz was much higher than that calculated at 10 GHz, which is presumably due to the Debye-type dielectric relaxation in GHz region. However, the dielectric loss of the samples with x = 0.6 and x = 0.7, extrapolated from IR range, was in agreement with that measured at MW frequency. Low-temperature Raman scattering showed that the band at 760 cm À1 assigned to the TiÀO 3 torsional mode markedly sharpens and shifts upward with increasing content of Sr. This is an indication that the bonding between cations and anions was tightened due to the substitution of Ba with Sr, which explains well the decrease in dielectric permittivity, loss and tunability with increasing concentration of Sr.