Monolithic samples of highly porous poly(styrene/DVB) PolyHIPE® have undergone a number of electrophilic aromatic substitution reactions, namely sulfonation, nitration and bromination. Mild, hydrophobic reagents and homogeneous reaction conditions were sought in an effort to achieve uniform chemical modification, to a reasonable degree of substitution, throughout the large polymeric structures. Thus, sulfonation was carried out with lauroyl sulfate in cyclohexane, nitration with tetrabutylammonium nitrate-trifluoroacetic anhydride (TBAN-TFAA) in dichloromethane and bromination with brominestannic chloride in dichloromethane. An average degree of sulfonation of 2.4 mmol g-1 was achieved, with a drop in sulfonic acid content of approximately 1 mmol g-1 from surface to centre. Nitration occurred to a lesser extent, with similar differences in substitution between surface and centre being observed. PolyHIPE® monolithic samples were brominated to an extent of 3.6 mmol g-1 furthermore, this was uniform across the entire substrate. The differences in extent of each reaction are explained by consideration of such factors as the nature of the solvent, polarity of the reagents and compatibility between the reagents and the polymer matrix throughout the reactions.