The effects of the variations in concentrations of hydrogen (0-480 mol/m(3)) and 1-butene (0-380 mol/m(3)) on the gas-phase copolymerization of ethylene and l-butene over a MgCl2/SiO2-supported Ti catalyst were investigated using a semibatch reactor operated at 70 degrees C. Polymers were characterized by size exclusion chromatography (SEC), melt now index, analytical and preparative temperature-rising elution fractionation (ATREF and PTREF), and PTREF-SEC cross-fractionation. Excellent correlations were obtained between the reactor operating conditions and polymer properties; e.g., the average polymerization rate was proportional to 1/(1 + a[H-2](0.5)), the methyl group concentration in the polymer was proportional to the l-butene concentration, and the melt flow index varied as M(w)(-3.4). The most significant finding was that the hydrogen concentration dependence of the termination rate by hydrogen was different for different catalytic sites; the termination rate was first order for the catalytic sites responsible fur the formation of copolymer and half-order for the sites responsible for the homopolymer component of the polymer.