The continuous countercurrent fractionation of fish oil ethyl esters using supercritical carbon dioxide is studied, modelling a process with internal reflux generated by a thermal gradient at the top stage. A methodology for process design is proposed and applied to determine the relationships between the temperature at the top stage (T-1), the number of theoretical stages (N), and the solvent to feed ratio (S IF), with the aim of providing a quantitative comparison with the external reflux process. The internal reflux process is viable and, for stated process specifications (mass fraction and recovery of C20 + C22 ethyl esters of 95%), provides comparable or better results than the external reflux process. For example, operating at 13.3 MPa and 50 degrees C, and keeping T-1 in the range (66-70) degrees C, the specifications are attained with N and S/F in the range 16-30 and 88-120, respectively. (C) 2013 Elsevier B.V. All rights reserved.