A commercial ethylene-butene linear low density polyethylene has been fractionated by preparative temperature rising elution fractionation. Each fraction was characterized by Fourier transform infra-red spectrometry to determine the branch content and by gel permeation chromatography to determine the molecular weight. Two fractions were then blended, in various proportions, with each other and with a linear polyethylene. Using differential scanning calorimetry and transmission electron microscopy to examine quenched blends, the phase behaviour of this ternary system was investigated at temperatures above the melting point. The phase behaviour is very similar to that previously found for a ternary system of a linear polyethylene with two ethylene-octene copolymers with branch contents close to those of the ethylene-butene copolymers used in the present work. We deduce that the phase behaviour of blends containing ethylene-butene copolymers is essentially the same as that of blends containing ethylene-octene copolymers. We further deduce that the length of the branches must be of secondary importance in determining phase behaviour. (Previously we have shown that the variation of molecular weight has a secondary effect on phase behaviour). Thus we conclude that in blends of linear polyethylene with lightly branched ethylene copolymers it is the number of branches that is the most important factor influencing the extent of phase separation in the melt.