In homogeneous Cr(III)methylaluminoxane/cyclopentyl-bisphosphineamine (PNP)/halide ethylene tetramerization catalyst systems, the effects of halide on the 1-octene formation selectivity and the catalytic activity were investigated. The comparative studies showed that both 1-octene formation selectivity and catalytic activity of the four-member catalytic systems containing dichloromethane were higher than those of containing trichloromethane and tetrachoromethane. 1,1,2-Trichloroethane show,ed much higher 1-octene formation selectivity improvement than 1,1,1-trichloroethane. The improvement of chloride on 1-octene formation selectivity and catalytic activity was much better than that of a corresponding bromide. So we can draw a conclusion that the steric hindrance, the molecular stability, the halides group configuration, and the types of the halides are important factors for ethylene tetramerization toward 1-octene. Some specific interaction modes of halogen groups with active Cr species in ylene tetramerization reaction. the catalytic cycle are proposed to explain the 1-octene selectivity improvement effects of halide in the eth (c) 2007 Elsevier B.V. All rights reserved.