The radii of gyration for a series of poly(ethylene-l-butene) copolymers of varying ethyl branch content were measured as a function of temperature in the melt via small angle neutron scattering (SANS), and their temperature coefficients, d In C-infinity/dT = kappa, changed from negative to positive. The characteristic ratio C-infinity decreased with increasing ethyl branch frequency. When possible, these results were compared with those obtained from Theta solvents and from rotational isomeric (RIS) state theory. The SANS-based chain dimensions were slightly larger than those obtained from Theta solvents. The values of kappa differed significantly from those extracted from Theta-solvent measurements. Serious discrepancies with RIS-based calculations were observed.