Far-infrared (F-IR) reflection spectra were measured for n-type Bi_0.88Sb_0.12, p-type (Bi_0.25Sb_0.75)₂Te₃, n-type Bi₂(Se_0.2Te_0.8)₃ and n-type Bi₂(Se_0.05Te_0.95)₃, and dispersion parameters, plasma frequency and damping factor were determined. For electrical conductivity, carrier density and mobility calculated from those dispersion parameters, good agreement was obtained with those from electrical measurements.