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Two isomorphous series Cu3SnS4-Cu2FeSnS4 (kuramite–stannite) and Cu2FeSnS4-Cu2ZnFeSnS4 (stannite–kesterite) have been studied by 57Fe and 119Sn Mössbauer spectroscopy. It has been found for the first series that at the Fe atom concentration below x0 = 0.45±0.02, iron atoms are only in the trivalent state and the concentration of Fe3+ and Cu1+ increases with an increase in x. The following scheme of isomorphous substitution of Cu for Fe may be proposed for this range of concentration: 2Cu2+(Td)→Cu1+(Td)+Fe3+(Oh). When x exceeds the boundary value x0, the Fe2+ ions appear. In the concentration range 0.451+(T4d)+Fe3+(Oh)→2Fe2+(Td). Unlike the first series with the heterovalent substitution, for the second series, the isovalent substitution is observed: Fe2+(Td)→Zn2+(Td). For the Cu2Fe1-xZnxSnS4 system, Fe3+ atoms appear for all compounds. The noticeable changes of Fe2+ and Fe3+ atom concentration were not observed with an increase in x. [ABSTRACT FROM AUTHOR]