Disclosed are processes for preparing a process for preparing a modified polyphenylene ether which comprises reacting any one of a borane compound having at least one boronhydrogen bond, an .alpha.,.beta.-unsaturated carbonyl compound or a mercaptane having a carbonyl group, a hydroxyl group or an alkoxysilyl group with a polyphenylene ether having a carbon-carbon unsaturated bond in a substituent, and optionally oxidizing the resulting borane-modified PPE under alkaline conditions.