Thin films of (Bi0.75Sb0.25)2Te3 of various thicknesses in the range 400-3200 Å have been prepared by flash evaporation at a pressure of 1 × 10-5 torr onto cleaned glass plates held at room temperature. X-ray Diffraction (XRD) and Transmission Electron Microscopy (TEM) revealed that the films were polycrystalline and the grain size increased with increasing thickness. Electrical resistivity was measured in the temperature range 300-450 K for different thickness films. From the temperature dependence of resistivity, semiconductor-like behaviour has been observed. Activation energy for conduction is found to be thickness dependent. Thickness dependence of activation energy arises due to the quantum size effect in the films. Thickness dependence of electrical resistivity has been analyzed using the effective mean free path model. From the analysis, important material constants like the mean free path and the electron concentration have been evaluated. © 1998 Elsevier Science Ltd. All rights reserved.