The invention relates to a process for separating dibutenes into an n-octene-containing fraction and a dimethylhexene-containing fraction. The fractions can be processed further separately to the corresponding C.sub.9 carboxylic acids and C.sub.9 alcohols (isononanols). Successor products of the C.sub.9 carboxylic acids include, for example, vinyl esters. Successor products of the C.sub.9 alcohols include plasticizers.