Different poly(ethene-co-1-hexene) samples with varying amounts of 1-hexene were characterized by crystallization analysis fractionation (Crystaf). The samples were synthesized with (1,2,4-Me3CP)(2)ZrCl2, (Me5CP)(2)ZrCl2, and a mixture of these two catalysts in a 1:1 molar ratio. In addition, preparative Crystaf was used to fractionate some of the samples made with the catalyst mixture into 1-hexene-rich and 1-hexene-poor fractions. These fractions were characterized by Crystaf, differential scanning calorimetry (DSC) and gel permeation chromatography (GPC), and compared with copolymers made under similar conditions using the individual catalysts. Both (1,2,4-Me3CP)(2)ZrCl2 and (Me5Cp)(2)ZrCl2 produced copolymers with unimodal distribution of short chain branches (SCBD), as expected for single-site catalysts. The catalyst mixture produced copolymers with bimodal SCBDs when 0.38 mol/l or higher concentrations of 1-hexene were used. The high temperature peak results from crystallization of polymer chains with few comonomer units, and these are attributed to (Me5CP)(2)ZrCl2. The low temperature peak results from crystallization of polymer chains made by (1,2,4-Me3CP)(2)ZrCl2, and these chains contain many comonomer units. Direct evidence for relative activity enhancement of the (Me5CP)(2)ZrCl2 catalyst in the dual-site system was observed. (C) 2004 Published by Elsevier Ltd.