In a process for co-oligomerization of olefins, an olefin starting material comprising olefins with n carbon atoms and olefins with 2n carbon atoms is converted over an olefin oligomerization catalyst to a reaction product. The process is performed under such conditions that the conversion of olefins with 2n carbon atoms is less than 10%. Both the co-oligomer with 3n carbon atoms and the olefin with 2n carbon atoms removed from the reaction product have a high hydroformylatability.