The present study reveals that the observed variation of thermal conductivity lambda with temperature, for any non-conducting linear amorphous polymer can be explained by considering phonons as chief heat carriers. It is proposed that in the temperature region below the glass transition temperature, T-g, structure scattering and chain-defect scattering are the predominant phonon scattering processes, while above T-g, vacant-site scatterings also play a significant role. Owing to the effect of temperature on the movement of structural units, the thermal resistances corresponding to these scattering processes have temperature dependence. Accordingly, relations for lambda can be deduced using Matthiessen’s rule. Calculated values of poly(vinyl chloride) with 0%, 10%, 20% and 40% plasticizers and several different styrene polymers are in excellent agreement with the reported experimental data, over a wide range of temperatures around T-g, maximum deviations being similar to 3%.