Molecular dynamics simulations and vibrational analysis on model amorphous polyethylene structures demonstrate that the main contribution to the coefficient of thermal expansion (CTE) is due to structural relaxations even at temperatures below the glass transition. The contribution due to vibrational relaxation, calculated within the Gruneissen theory of solids, accounts only for a small fraction of the CTE. Based on our findings we argue that tailoring of polymers with low CTE should focus on reduction of the relaxation processes. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2114-2121, 2009