The present invention provides a method for increasing hydrocarbon chain length. The method comprises providing a feedstock comprising fatty acids and/or fatty acid esters with hydrocarbon chain length below C23 into a reaction zone in which ketonization is conducted in the presence of a hydrotreatment catalyst under hydrogen pressure. The obtained ketones have a hydrocarbon chain length of from C24 to C43. The present invention further provides a method for simultaneous production of base oil components and fuel components.