Phase formation studies in the quaternary section Ca_2-<I>y</I>Sr<I>_y</I>Fe_2-<I>x</I>B'<I>_x</I>O<I>_γ</I> : B' = Nb, Ta, 0 ≤ <I>y</I> ≤ 2.0 and 0 ≤ <I>x</I> ≤ 2.0 were carried out. Results are summarised below for B' = Nb; for B' = Ta they were broadly similar. Four solid solution phases were found: i) a cubic solid solution, with a variable compositional extent <I>x</I>, which increased with substitution of Ca by Sr, from 0.45 ≤ <I>x</I> ≤ 0.65 for <I>y</I> = 0, to 0 ≤ <I>x</I> ≤ 1.2 for <I>y</I> = 2.0. ii) A limited orthorhombic solid solution along the Ca₂Fe₂O₅-Ca₂B'₂O₇ join with 0.8 ≤ <I>x</I> ≤ 1.0. iii) A complete range of orthorhombic, brownmillerite solid solution for <I>x</I> = 0 and iv) an orthorhomic, perovskite-related solid solution, isostructural with Ca₂Nb₂O₇, for <I>x</I> = 2.0. A combination of differential thermal analysis, thermogravimetry, Mössbauer spectroscopy, high temperature powder X-ray diffraction, Rietveld refinement, a.c. impedance spectroscopy and magnetic susceptometry has been used to characterise these new phases. Solid solution (i) has variable oxygen content with an oxygen-deficient perovskite structure. As synthesised, for <I>x</I> = 0.6 it contains a mixture of Fe⁴⁺ and Fe³⁺ in the ratio 13/87. The oxygen content could be modified by heat treatment under various atmospheres over the range 5.5. ≤ <I>γ</I> ≤ 5.92 which corresponded to the ratios: 93/7 to Fe³⁺/Fe²⁺ and 42/58 of Fe⁴⁺/Fe³⁺. At higher oxygen contents, <I>γ</I> > 5.7, transformation to an orthorhombic structure was observed. The electrical resistivity was dominated by grain boundary effects. Conduction is electronic and is attributed to hopping between iron in mixed oxidation states. The resistivity increased dramatically on reduction and became <I>n</I>-type, presumably due to a small fraction of Fe²⁺ present.