An influence of Te doping on the lattice parameter and the thermal expansion of AlxGa1−xAs epitaxial layers was examined by high‐resolution x‐ray diffractometry over a temperature range 77–673 K. For doped AlxGa1−xAs layers the thermal expansion coefficients were found to be larger relative to undoped samples of the same Al content. This phenomenon is attributed to the change of anharmonic part of lattice vibrations by free electrons and/or ionized tellurium atoms. An increase of thermal expansion caused by doping is a factor which should be taken into account in lattice constant measurements at 295 K. We propose an interpretation of the lattice expansion (examined at room temperature by other authors and by us) of GaAs caused by Te doping in terms of three factors: (i) ‘‘size’’ effect (bigger Te atoms with respect to As), (ii) free electrons via the deformation potential of the conduction‐band minimum occupied by these electrons, (iii) increase of the thermal expansion. ©1995 American Institute of Physics.