Twenty-two Mn II complexes were screened for the catalytic epoxidation of terminal olefins using peracetic acid as the oxidant. A limited number of these complexes are efficient catalysts using peracetic acid solutions generated with H 2 SO 4 (PAA C ), but most complexes are effective at 1mol% catalyst loading using peracetic acid generated with strongly acidic resins (PAA R ). Under the less acidic conditions of PAA R , [Mn II (phen) 2 (CF 3 SO 3 ) 2 ] has the highest activity of the catalysts screened, and epoxides terminal olefins using as little as 0.02mol% catalyst within 5min. The dimeric species [Mn III,IV 2 (phen) 4 (O) 2 ](ClO 4 ) 3 is also a viable epoxidation catalyst with PAA R , but the dimeric species is reduced by the residual H 2 O 2 to monomeric Mn II species under the reaction conditions. By comparison, a similar dimeric complex [Mn III,IV 2 (R,R-mcp) 2 (O) 2 ](ClO 4 ) 3 , is not reduced by H 2 O 2 under the reaction conditions and is not catalytically active, supportive of the notion that the catalytically relevant species is monomeric.