We have investigated a new semiconducting material Pb1-x(Mg1-ySry)(x)S, a quaternary solid solution compound. The experimental results obtained on bulk samples heat-treated under thermal equilibrium show that the lattice constants and the energy band gaps at room temperature of the Pb1-x(Mg1-ySry)(x)S system change from 5.862 to 6.120 Angstrom and from 0.41 to around 0.9 eV, respectively, with changes of x. This suggests that the Pb1-x(Mg1-ySry)(x)S compounds have a potential for wide wavelength coverage from mid-infrared to near infrared if the quaternary solid solution semiconductor is used as the active layer in a laser diode. The solubility limit of x in the Pb1-x(Mg1-ySry)(x)S system along the line of lattice-matching to a PbS is 0.17 at 1000 degreesC, and the difference in the energy band gap between the Pb1-x(Mg1-ySry)(x)S and the PbS is 0.46 eV in maximum at room temperature.