The absorption diffusivity of dicumyl peroxide (DICUP) in low density polyethlene pellets that have been annealed and either cooled slowly or quenched has been measured and correlated with the corresponding changes in crystallinity as measured by DSC. Annealing at 125°C resulted in an increased crystallinity and decreased DICUP diffusivity (3.2 × 10−7 cm2/s for as supplied pellet to 5.2 × 10−8 cm2/s for annealed and slowly cooled pellets). Quenching the annealed pellet reduced the increase in crystallinity and corresponding decrease in diffusivity. The time course of annealing and secondary crystallization processes was defined through further DSC measurements. The relationship between crystallinity and pressure for both hot press films and 4 mm diameter extrudate was found to be identical, Indicating the importance of pressure on bulk crystallization phenomena regardless of the source of the pressure.