This paper presents the results of an experimental study of diffusion of 2,4,6-trichlorophenol, 2,3,5,6-tetrachlorophenol, and pentachlorophenol through two 17-year-old high-density polyethylene geomembranes (HDPE GMs): the first was stored indoors away from direct sunlight whereas the second was exposed for the entire time in a south-facing corner of a pond. Both unexposed and exposed specimens, although under different exposure conditions, experienced ageing. Partition coefficients ranged from 30 to 190, increasing with the number of chlorine atoms in the contaminant. Diffusion coefficients for the unexposed geomembrane ranged from 9 x 10(-14) to 2.4 x 10(-13) m(2)/s and from 9 x 10(-14) to 2.8 x 10(-13) m(2)/s for the exposed geomembrane. These values were compared with those reported in the literature for diffusion of the same phenolic compounds through a virgin HDPE geomembrane from a different manufacturer: the exposure did not noticeably affect the partition coefficients or diffusion coefficients. This result was linked to the temporal evolution of the unexposed geomembrane. Furthermore, in spite of their 17-year ageing, both geomembrane specimens still had diffusive characteristics that were consistent with a virgin geomembrane obtained from a different manufacturer.