It has been found that the activity of a catalyst for selective oligomerization (trimerization) of ethylene based on a chromium salt and organoaluminum compounds (OACs) can be substantially enhanced by the microwave irradiation of organoaluminum compounds directly prior to mixing them with the chromium salt and a pyrrole ligand. The effects of irradiation conditions and the nature of the reagents used have been studied. The catalyst obtained according to an optimal procedure outperforms known analogs in activity.