The phase behaviour in the melt in blends of a single-site linear polyethylene and several single-site branched ethylene-l-alkene copolymers is examined, using an indirect technique based on examination of rapidly quenched melts in the solid state using differential scanning calorimetry, transmission electron microscope and atomic force microscopy. The extent of phase separation is found to have increased if the amount of comonomer in the branched blend component is increased. This result is observed for ethyl and butyl type short chain branches and found to be valid as long the amount of comonomer is less than approximately 5.0 mol%. For higher amounts of comonomer incorporation, the extent of phase separation is found to have approximately fixed. These observations might be partly predicted from theory if an extra repulsive potential is added to the ordinary Flory-Huggins equation.