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 aluminium trihalide; and c) at least one organic Bronsted acid; the catalytic composition being pre-conditioned in a solvent before using it for oligomerization.