Oxidation of dodecacarbonyltriruthenium with halogens (X2; X = Cl, Br, or I) gives the monomeric cis-Ru(CO)4X2, the dimeric Ru2(CO)6X4, the trimeric Ru3(CO)12X6 or the polymeric [Ru(CO)2X2]n depending upon the reaction conditions. On the basis of i.r. studies, it would appear that the initial product in all of these oxidation is cis-Ru(CO)4X2 with subsequent polymerisation to give the dimeric and the trimeric compounds. No evidence for the formation of trans-Ru(CO)4X2 has been found. The dimeric, trimeric, and polymeric compounds are also produced from the reaction of the ruthenium trihalides (RuX3) with carbon monoxide at atmospheric pressure and moderate temperatures. I.r. and mass spectral data are reported and various structural possibilities for these compounds are discussed. Oxidation of (Ph3P)3Ru3(CO)9 with halogens give the mononuclear derivatives (Ph3P)Ru(CO)3X2. These are unstable in solution and decompose with loss of carbon monoxide to give the dinuclear compounds [(Ph3P)Ru(CO)2X2].