The elusive mono-hydride tri-anion [HRu4(CO)12]3- (4) has been isolated and fully characterized for the first time. Cluster 4 can be obtained by the deprotonation of [H3Ru4(CO)12]- (2) with NaOH in DMSO. A more convenient synthesis is represented by the reaction of [HRu3(CO)11]- (6) with an excess of NaOH in DMSO. The molecular structure of 4 has been determined by single-crystal X-ray diffraction (SC-XRD) as the [NEt4]3[4] salt. It displays a tetrahedral structure of pseudo C3v symmetry with the unique hydride ligand capping a triangular Ru3 face. Variable temperature (VT) 1H and 13C{1H} NMR experiments indicate that 4 is fluxional in solution and reveal an equilibrium between the C3v isomer found in the solid state and a second isomer with Cs symmetry. Protonation-deprotonation reactions inter-converting H4Ru4(CO)12 (1), [H3Ru4(CO)12]- (2), [H2Ru4(CO)12]2- (3), [HRu4(CO)12]3- (4) and the purported [Ru4(CO)12]4- (5) have been monitored by IR and 1H NMR spectroscopy. Whilst attempting the optimization of the synthesis of 4, crystals of [NEt4]2[Ru3(CO)9(CO3)] ([NEt4]2[7]) were obtained. Anion 7 contains an unprecedented CO32- ion bonded to a zero-valent Ru3(CO)9 fragment. Finally, the reaction of 6 as the [N(PPh3)2]+ ([PPN]+) salt with NaOH in DMSO affords [Ru3(CO)9(NPPh3)]- (9) instead of 4. Computational DFT studies have been carried out in order to support experimental evidence and the location of the hydride ligands as well as to shed light on possible isomers.