Thin films (Bi0.25Sb0.75)2Te3 alloy of thickness in the range 400-2200 Å have been deposited on clean glass substrates by the flash evaporation technique in a vacuum of 1 × 10-5 Torr. Investigation by X-ray diffraction (XRD) and transmission electron microscopy (TEM) showed that all the films were polycrystalline and the grain size increased with increasing film thickness. Annealing treatment leads to grain growth. Electrical resistivity and thermoelectric power measurements were carried out for different thicknesses of the films in the temperature range 300-450 K. The dependences of resistivity and thermoelectric power on the temperature show that (Bi0.25Sb0.75)2Te3 films are semiconducting. Least square fit of electrical resistivity and thermoelectric power data with reciprocal thickness indicates that the results on (Bi0.25Sb0.75)2Te3 alloy films agree with the prediction of size effect theories. The effective mean free path model of size effect with perfect diffuse scattering is used for the analysis of the data. From the fit, the important physical parameters i.e., mean free path (lg), bulk resistivity (ρg), bulk thermoelectric power (Sg) and energy dependent mean free path relation power index (Ug) have been evaluated. © 1998 Elsevier Science S.A. All rights reserved.