The transition metal carbonyl hydrides H3Re3(CO)12, H4Re4(CO)12, H4Ru4(CO)12, and H4Os4(CO)12 have been formed in high yields and purity by treatment of the corresponding metal carbonyls [Re2(CO)10, Ru3(CO)12, and Os3(CO)12] at temperatures of 90–170° in hydrocarbon solvents with hydrogen at atmospheric pressure; similar conditions convert H2Ru4(CO)13 into H4Ru4(CO)12 and H2FeRu3(CO)13 into H4FeRu3(CO)12, a new but unstable species.