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Abstract: Using first-principle method, we investigate the structural, electronic, optical and thermodynamic properties of the strontium semiconductors Pb1− x Sr x S with 25%, 50% and 75% of Sr. The calculations are performed by using the full-potential linearized augmented plane wave (FP-LAPW) method. As exchange–correlation potential we used the generalized gradient approximation (GGA) of Perdew et al. The variation of the calculated equilibrium lattice constant versus concentration shows that a small deviation from Vegard’s law is clearly visible with downward bowing parameter equal to 0.009Å. The bulk modulus as a function of x for Pb1− x Sr x S alloy shows a significant deviation from the linear concentration dependence (LCD) with downward bowing equal to 6.62GPa. The different roles of structural and chemical effects on the gap bowing and its variation with composition are identified and discussed. In addition, density of states and optical properties were calculated and compared to the available experimental data and previous theoretical works. On the other hand, the thermodynamic stability of these alloys was investigated by calculating the excess enthalpy of mixing ΔH m as well as the phase diagram. [ABSTRACT FROM AUTHOR]