The purpose of this work was to investigate the effect of solute size on transport in structured porous media. Miscible displacement experiments were performed with tracers of different sizes 3H2O), pentafluorobenzoate (PFBA), 2,4-dichlorophenoxyacetic acid (2,4-D), and hydroxypropyl-&bgr;-cyclodextrin (HPCD)> in aggregated, stratified, and macroporous media. The breakthrough curves exhibited both early breakthrough and tailing, indicative of nonideal transport in these structure media. Comparison of breakthrough curves revealed that the extent of nonideality (e.g., tailing) was HPCD>PFBA, 2,4-D>3H2O. This behavior is consistent with the impact of solute size on the relative degree of ''nonequilibrium'' experienced by solutes whose transport is constrained by diffusive mass transfer. The capability of the first-order, dual-porosity mobile-immobile model to represent solute transport in these structured systems was evaluated by comparing independently determined values of the input parameters to values obtained by curve fitting of the experimental measurements. The calculated and optimized values compared quite well for the aggregated and stratified media, but not for the macroporous media. Experiments performed with tracers of different size are useful for characterizing the nature of the porous medium through which transport is occurring. ¿ American Geophysical Union 1995