In the presence of a template, desmetryn, commercial porous polypropylene membranes were photografted, using benzophenone as photoinitiator, with the functional monomer 2-acrylamido-2-methylpropanesulfonic acid and the cross-linker N,N'-methylenebis(acrylamide) in water. The influence of the polymerization conditions on membrane properties was studied by solute sorption analyses and membrane permeability measurements as well as electron spectroscopy for chemical analysis, FTIR spectroscopy, BET analysis, and scanning electron microscopy. It was found that molecularly imprinted polymer (MIP) membranes can be obtained which possess group affinity for the template and other triazine herbicides. Remarkably, the MIP membrane affinity for the template in buffer solution can be improved by the presence of salt during photograft copolymerization. This work presents the first successful example for molecular imprinting by in situ polymerization in water and on the surface of a commercially available synthetic polymer. The novel MIP membranes can be used in a fast preconcentration step, solid-phase extraction, by a simple microfiltration for the determination of herbicides in water. The possibility to introduce specific binding sites into porous membranes by surface imprinting polymerization without reducing their high permeabilities opens a general way to design new high-performance affinity membranes.