This paper presents experimental results of the diffusion of phenolic compounds through a flexible polypropylene (f-PP) geomembrane, two co-extruded ethylene vinyl alcohol (EVOH) geomembranes, and a high density polyethylene (HDPE) geomembrane. The study considers the partition and diffusion of two chlorophenol contaminants (2,4,6-tricholophenol and 2,3,5,6-tetrachlorophenol) that are known to be toxic even at very low concentrations. The concentration dynamics in the source and receptor chambers of the diffusion cells were interpreted using the numerical code Pollute. No significant differences could be noted between the various HDPE geomembranes, whether or not a thin EVOH film was co-extruded with HDPE. Diffusion through the f-PP geomembrane was significantly larger, by an order of magnitude, when one focuses on the permeation coefficient.