Butylacrylate – styrene co-polymers prepared by atom transfer radical polymeratization were separated on an octadecyl silica column by gradient elution with tetrahydrofuran in water, up to the molar masses 10,000. In reversed-phase high performance liquid chromatography (RP-HPLC), the retention of macromolecules is affected very significantly even by change of a few tenths of per cent of the organic solvent in the aqueous-organic mobile phase. Therefore, gradient elution was used for the determination of the parameters of the equations describing the effects of the mobile phase on the retention behaviour of synthetic polymers. The retention parameters of homopolymers and copolymers were calculated from the gradient data using two retention models. The retention behaviour of the copolymers was described using the experimental gradient retention data for homopolymers.