High Quality Content by WIKIPEDIA articles! Methylcyclopentadienyl manganese tricarbonyl (MMT or MCMT) is an organomanganese compound with the formula (CH3C5H4)Mn(CO)3. Marketed as a supplement to the gasoline additive tetraethyllead to increase a fuel`s octane rating, MMT was later used in unleaded gasoline. Following the implementation of the Clean Air Act (United States) (CAA) in 1970, MMT continued to be used alongside TEL in the US during the phase-out of leaded gasoline (prior to TEL being finally banned from US gasoline in 1995) and was also used in unleaded gasoline until 1977. Ethyl Corporation obtained a waiver from EPA in 1995. The waiver allows the use of MMT in US unleaded gasoline (not including reformulated gasoline) at a treat rate equivalent to 8.3 mg Mn/l.