The X-ray analyses of two crystalline modifications, (I) and (II), of [N(PPh₃)₂][HRu₆(CO)₁₈] are reported, together with improved synthetic routes to this and the related clusters [Ru₆(CO)₁₈]^2– and H₂Ru₆(CO)₁₈. Crystals of (I) are triclinic, space group P₁^–, with a= 18.083(4), b= 19.101(4), c= 19.238(5)Å, α= 117.70(4), β= 78.13(2), γ= 97.05(2)°, and Z= 4. Crystals of (II) are monoclinic, space group P2₁/n, with a= 33.82(8), b= 52.55(10), c= 9.832(2)Å;, β= 92.66(2)°, and Z= 12. Least-squares refinement using diffractometer data (Mo-K_α) has given an R of 0.068 1 for 9 165 reflections for (I) and an R of 0.23 (Ru only) for 1 485 reflections for (II). The unit cell in (I) contains two independent molecules of [HRu₆(CO)₁₈]^–, cluster (1) which is ordered and cluster (2) which is disordered between two sites (2A) and (2B) that are related by a non-crystallographic two-fold axis. The combined evidence of the X-ray analyses, ¹H n.m.r. studies, i.r. [v(CO)] spectra, and variable-temperature ¹³C n.m.r. is only consistent with the hydrogen ligand lying inside the Ru₆ octahedron.