Polycrystalline samples of layered perovskite-related La 2 Ru 1−y Mn y O 5 (0 y 0.25) were prepared by a soft-chemistry synthesis based on the thermal decomposition of a citrate precursor. The crystal structures of the compounds were determined by Rietveld analysis of x-ray diffraction patterns recorded at different temperatures. Additional neutron diffraction patterns for La 2 Ru 0.75 Mn 0.25 O 5 were recorded between 1.5 K and 300 K. A structural phase transition from a monoclinic room-temperature to a triclinic low-temperature phase was observed for y 0.20, while for y = 0.25 a phase separation at low temperatures was identified. The structural phase-transition temperatures decrease with increasing Mn concentration. Selected area electron diffraction of La 2 Ru 0.75 Mn 0.25 O 5 documented the absence of cationic ordering of Ru and Mn. The Mn and Ru oxidation states were determined using x-ray absorption spectroscopy and showed a stable +4 valence for both elements in all samples investigated, at room temperature as well as at 110 K.