Abstract The magnetic and low temperature structural properties of the one dimensional organic charge transfer salts β-(TMTSF)2 Re6Se5Cl9 and β-(TMTTF)2 Re6Se5Cl9 are investigated. It is found that the former compound exhibits a spin-Peierls-like transition at ca. 65 K resulting in a tetratnerization of the donor stack. No such a phase transition is observed in the TMTTF salt down to 20 K. This is ascribed to a difference in their structure, namely a stronger donor-anion (Se…Se) interaction in the selenium analogue.