The limits of detection of phase separation in linear/branched polyethylene systems have been investigated, using the very sensitive but indirect experimental methods developed at the University of Bristol. A linear polyethylene has been blended with a number of very lightly branched, ethylene-octene and ethylene-butene copolymers (containing between 0.7 and 3.1 mol% of copolymer). Samples of blends and of mixtures of the components were examined by differential scanning calorimetry (d.s.c.) and by transmission electron microscopy (TEM). Results indicate that TEM is more sensitive than d.s.c. in detecting phase separation in quenched linear/lightly branched copolymer systems. TEM can detect phase separation in blends when the blended copolymer contains just below 1 mol% of either butene or octene comonomer. D.s.c. requires a higher comonomer content, just over 1.5 mol% of octene and somewhere between 1.3 and 3 mol% of butene comonomer. For both techniques, phase separation can be detected at similar levels of branching for blends containing linear polyethylene with ethylene/octene comonomer and for blends containing linear polyethylene with ethylene/butene comonomer.