The complexes [Os₄(CO)₁₁(cyclo-olefin – H)H₃](cyclo-olefin = cyclo-octatetraene, cyclo-octa-1,5-diene, cyclo-hexene, or norbornene) have been identified as the products of reaction of [Os₄(CO)₁₂H₄] with the appropriate cyclic olefin. The structures of these complexes are discussed in terms of available spectroscopic data and in the case of the title complex [Os₄(CO)₁₁(C₆H₉)H₃] on the basis of a single-crystal X-ray analysis. This molecule crystallises in space group P2₁/n with a= 9.266(4), b= 14.850(8), c= 16.847(9)Å, β= 93.39(3)°, and Z= 4. The osmium atoms define a tetrahedron, with the cyclohexene ring σ-bonded to one metal and π-bonded to another. The complex [Os₄(CO)₁₂H₄] also reacts with the acyclic olefins CH₂CH₂, CH₂CHPh, CH₂CHCMe₃, CH₂CMeCMe₃, and cis-CHPhCHPh to give the related complexes [Os₄(CO)₁₁(olefin – H)H₃]. On heating, apart from the complex where the olefin is CH₂CMeCMe₃, dehydrogenation occurs to produce the dihydrido-complexes [Os₄(CO)₁₁(R¹CCR²)H₂](R¹= H, R²= H, Ph, or CMe₃; R¹= R²= Ph). These complexes have been shown to contain a R¹CCR² fragment bound via two σ and one π bond to one tetrahedral face of the Os₄ cluster unit.