The results of a magneto chemical and Mössbauer characterization are reported for the solid solution Sr3Ti2-xFexO7-δ (x ≤ 0.5), the intergrowth of a double perovskite block and one rock-salt layer type. The charge compensation mechanism induced by the introduction of iron atoms in the matrix of Sr3Ti2O7 is sensitive to the conditions of synthesis, namely an oxidation process triggers the formation of mixed Fe(III)-Fe(IV) valences. The crystallographic characterization - variation of the cell parameters and structure calculations – brings evidence for the respective occurrence of mixed valences and oxygen vacancies which form in the middle plane of the double perovskite block. Ferromagnetic exchange interactions which are absent in the Fe(III) containing compositions, appear and progressively strengthen depending on the oxidizing treatment. They are ascribed to the presence of an increasing amount of Fe(IV) species. Remarkably, a mixed valence state of iron forms during annealing in air with an increasing contribution of the Fe(IV) species for the larger iron contents, as deduced from Mössbauer data.