A liquid-phase process for polymerizing .alpha.-olefins of the formula CH.sub.2.dbd.CHR, where R is H or an alkyl radical C1 C6, to produce a polymer that is soluble in the reaction medium, comprising the steps of: a) continuously polymerizing in liquid phase the .alpha.-olefin in the presence of a catalyst system based on a transition metal compound; b) continuously withdrawing from step a) a solution of polymer in the liquid reaction medium; c) mixing in one or more mixing stages said solution of polymer in the reaction medium with an organic deactivator having: at least a hydroxy group, a boiling point higher than 150.degree. C., and a ratio between the molecular weight (MW) and the number of hydroxy groups (OH) comprised between 20 and 100.