The strong interplay between magnetism and transport can tune the thermoelectric properties in chalcogenides and oxides. In the case of ferromagnetic CoS2 pyrite, it was previously shown that the power factor is large at room temperature, reaching 1 mW m−1 K−2 and abruptly increases for temperatures below the Curie transition (TC), an increase potentially due to a magnonic effect on the Seebeck (S) coefficient. The too large thermal conductivity approximately equal to 10.5 W m−1 K−1 at room temperature prevents this pyrite from being a good thermoelectric material. In this work, samples belonging to the Co1−xFexS2 pyrite family (x = 0, 0.15 and 0.30) have thus been investigated in order to modify the thermal properties by the introduction of disorder on the Co site. We show here that the thermal conductivity can indeed be reduced by such a substitution, but that this substitution predominantly induces a reduction of the electronic part of the thermal conductivity and not of the lattice part. Interestingly, the magnonic contribution to S below TC disappears as x increases, while at high T, S tends to a very similar value (close to −42 µV K−1) for all the samples investigated. This article is part of a discussion meeting issue ‘Energy materials for a low carbon future’.