Rezanezhad, F., Price, J. S. and Craig, J. R. 2012. The effects of dual porosity on transport and retardation in peat: A laboratory experiment. Can. J. Soil Sci. 92: 723-732. Peatlands cover up to half of the local landscape in the Athabasca oil sands region, and peat materials are commonly used in reclamation. The toxicity of oil sands process-affected water (OSPW) derived from the Athabasca oil sands is related to the elevated concentration of naphthenic acids (NAs) and sodium (Na). However, the transport and retardation of solutes through peat is significantly delayed by sorption and by diffusion into immobile water contained in the peat matrix. Approximately 94% of the 43.5 mg L^-1 of OSPW was sorbed by 1 kg of peat. For Na ~84% sorption occurred with 382 mg L^-1 kg^-1 of peat. The sorption and desorption of NAs and Na on peat fitted linear isotherms with distribution coefficients of 6.53 and 5.74 L kg^-1, respectively. Solute breakthrough tests were performed for NaCl and the retardation due to sorption (R) of 1.73 was estimated for Na using a two-region (mobile and immobile) non-equilibrium transport model. However, the estimated mass transfer coefficient describing solute exchange between the mobile and immobile liquid regions indicated that part of retardation is attributed to solute exchange between the mobile and immobile phases. This was evident because Cl, a conservative solute (R=1, no sorption), also exhibited characteristics of dual-porosity transport. Thus, the passage of the solute front was retarded by diffusion of solute into the immobile region and by solute exchange between the mobile and immobile phases, which occurs at a rate depending on the proportion of mobile/immobile pore spaces. In this study, we showed that the complex dual-porosity structure of the peat is the important factor in attenuating solute transport where the presence of immobile phase (dead-end pores) in the system contributes to the transport and sorption mechanism of solute into this porous medium.