An improved catalytic composition for oligomerization, in particular
dimerization, of monoolefins comprises the product resulting from
bringing the following three constituents into contact in any order: a)
at least one divalent nickel compound; b) at least one
hydrocarbylaluminium dihalide, optionally enriched with an aluminum
trihalide; and c) at least one organic Bronsted acid;the catalytic
composition being pre-conditioned in a solvent before using it for
oligomerization.