The extensive initial loss of efficiency of antioxidant [4,4’-thiobis(3-methyl-6-tertbutylphenol)], during hydrostatic pressure testing of medium density polyethylene pipes at 95 and 105 degrees C has been studied. Samples from the pressure-tested pipes were subsequently investigated by scanning electron microscopy and energy dispersive spectroscopy. Antioxidant-containing particles were found in the pipe wall. The particles had evolved during the early stages of exposure and consisted of antioxidant that had phase separated and precipitated from the polymer matrix because the initial antioxidant concentration was greater than the solubility limit at the pressure testing temperatures. Relative antioxidant concentration profiles over the pipe wall, as measured by calorimetric oxidation induction time analysis, showed losses of >80% of the initial effective antioxidant content during the first 1000 h of pressure testing. A model assuming diffusion of antioxidant into precipitates was developed. Values for the diffusivity of the antioxidant in different parts of the pipe wall were obtained by fitting a previously developed model to long-term antioxidant concentration profiles. Using these values, and by using the distance between two adjacent precipitates as an adjustable parameter, it was possible to fit the newly developed model to the experimental short-term antioxidant concentration profiles.