In this Article, we elucidate the structural and thermoelectric properties of stannite-kuramite solid solutions, Cu2+xFe1-xSnS4-y), (x = 0-1), with sulfur defects (y) <= 0.4. Structural analysis revealed that anisotropy decreases and Cu/Sn disorder increases with an increase in x. The samples with x = 0.8-1 exhibit degenerate conduction, whereas the Seebeck coefficient (S) remains relatively high, S approximate to 100 mu V K-1 for x = 0.8 at 300 K. Thermal conductivities (K) of the solid solutions are in the range 10(-3)-10(-2) W cm(-1) K-1, which is close to the K value of silicon dioxide. The dimensionless figure of merit (ZT) reaches 0.044 for x = 0.8 at 300 K. The ZT is enhanced significantly by that of x = 0 at 300 K. These findings allow us to attain higher ZT values an increase in temperature and is doubly larger than that x=0 at 300 K. These findings allow us to attain higher ZT values through optimization of chemical composition.