We have investigated the optical and electrical properties of Bi- or Tl-doped Pb1-x(Mgl-ySry)(x)S thin films. The films with carrier concentration sufficient for use as cladding layers in a laser diode, were obtained by doping with Bi as n-type impurity or Tl as p-type impurity. Optical absorption was observed at far-infrared region on the Bi-doped films. and the relation between absorption coefficient alpha and carrier concentration n was found to be expressed as alpha = 0.27n(0.21) at 6 mum. (C) 2004 Elsevier B.V. All rights reserved.