The copolymerization of ethylene and 1-hexene over a spherical polymer/MgCl2-supported TiCl4 catalyst was studied as a function of the polymerization temperature from 40 to 100 ° C in a slurry reactor and from 120 to 200 ° C in a solution reactor with triethylaluminum (TEA) as a cocatalyst (1.0-6.8 mmol). The activities increased from 40 to 80 ° C and then declined monotonically with increases in the temperature during the slurry and solution polymerizations. The kinetic behavior in the slurry and solution operations was described by the same rate expression. The modeling results indicated that the catalyst had at least two different types of catalytic sites; one site was responsible for the acceleration-decay nature of the activity profiles, whereas the second site resulted in long-term activity. The apparent activation energy for site activation in the slurry operation was 69.9 kJ/mol; no activation energies for site activation could be estimated for the solution operation because the activation process was essentially instantaneous at the higher temperatures. The activation energies for deactivation were 100.3 kJ/mol for the slurry operation and 31.2 kJ/mol for the solution operation. The responses to TEA were similar for the slurry and solution operations; the rates increased with increasing amounts of TEA between 1.0 and 3.4 mmol and then decreased with larger amounts of TEA. © 2005 Wiley Periodicals, Inc.