The relaxation phenomena of two semicrystalline polymers, polyoxymethylene and poly(ethylene oxide), were studied by a dilatometric technique in the temperature range—30 to 200°C. Polyethylene was also investigated, in order to compare property-structure relations. The premelting (Tαc) and upper glass [Tg(U)] transitions are observed in all three polymers. Comparison of our results with previous work, at higher frequencies in most instances, as well as thermal treatment of one of our samples (polyoxymethylene) allow us to make the assignments and calculate the activation energies for the relaxations. Certain characteristic quantities are given: [T < Tg(L)/Tg(L)/], Tg(L)/Tm, and Tαc/Tm, as well as ΔαTgand αΔg.