Iron(II) meso-tetraphenylporphyrin is an efficient catalyst for the selective olefination of a variety of aromatic and aliphatic aldehydes with use of ethyl diazoacetate in the presence of triphenylphosphine. These reactions gave olefin products in excellent yields (>85%) with high selectivity for the E-isomer (>90%). For the olefination of ketones, the reactions were generally slow and the selectivities were low compared with those observed with aldehydes. Iron(III)meso-tetraphenylporphyrin chloride, reduced in situ, could be used as an olefination precatalyst to produce similar yields and selectivities. The olefination mechanism was investigated and the likely pathways are discussed.