The present application refers to a process for preparing 6,10- and 6,9-dimethyl-5,10-undecadienyl-2-ones by telomerizing isoprene with alkyl acetoacetates in the presence of a catalyst system containing a transition metal compound catalyst and a phosphorus- or arsenic-containing compound cocatalyst, wherein the process is performed in the presence of a protic additive of the general formula in which: R1, R2, R3 represent an alkyl group with 1 to 9 carbon atoms, branched or unbranched, a cycloalkyl group with 6 to 8 carbon atoms, an aryl group, in particular phenyl, or an arylalkyl group, X(−) represents an inorganic or organic anion, y is 1 or 2, and the &bgr;-ketoester so obtained is saponified and decarboxylated in a manner known per se.