Rotational weak-field magnetoresistance measurements at 78 K were used to determine the parameters of the valence band of light holes for Pb0.8Sn0.2Te and Pb0.8Sn0.2Te0.98S0.02 epitaxial films grown on (111)-oriented BaF2. It was shown that an isovalent substitution of 2% of tellurium atoms by sulphur atoms in solid solution films of lead telluride-tin telluride leads to the decrease of the energy splitting value of valence band valleys Delta epsilon(v) from 18 meV to 2 meV. The physical reasons for this phenomenon were analysed, Obtained experimental data were compared with the results of other experiments on photoluminescence and concentration dependencies of thermoEMP coefficient.