We have investigated the fabrication of Pb1-chiMgchiS thin films on BaF2 substrates by using hot-wall epitaxy. Mg concentration chi and its deviations from stoichiometry in the Pb1-chiMgchiS thin films were controlled by monitoring the growth conditions. We found a widened solubility range of Mg in the Pb1-chiMgchiS thin film as compared to the same range obtained for bulk growth. The values of the energy band gap of the films increased with increasing chi. The relation between the energy band gap E (g) and the composition chi of the Pb1-chiMgchiS thin films follows the expression E (g) = 0.417+ 1.111 chi -0.861 chi (2) (chi less than or equal to0.12). The information obtained here will be useful for future preparation of Pb1-chiMgchiS1-gammaSegamma or Pb1-chi(Mg1-gammaSrgamma)(chi)S quaternary films for fabrication of mid-infrared lasers.