A novel method to prepare a three-constituent adduct of metallocene CP2MCl2/2LiCl/THF is proposed. The adduct catalyst can be easily prepared without salt separation and recrystallization and possessed a low moisture sensitivity and a high stability. Measurements with NMR and XPS showed that specific intermolecular interactions exist among the three components, by which the stability of the adduct was increased. The activity of the catalyst was evaluated in the polymerization of ethylene. The kinetic behavior was also studied. It showed a high reservation of activity even when exposed to a high relative humidity for hours. (c) 2006 Wiley Periodicals, Inc.