Molecularly imprinted polymer membranes for a model compound, atrazine, were prepared in situ from a monomer mixture containing methacrylic acid, tri(ethylene glycol) dimethacrylate, and atrazine using U-V-initiated polymerization. To improve flexibility and mechanical stability of these membranes, oligourethane acrylate was added to the mixture of monomers. Polymeric additives were used to increase porosity of membranes and their permeability as well as to make them suited for filtration experiments. This process resulted in the formation of thin, flexible, and porous membranes containing atrazine-specific binding sites. The atrazine-imprinted membranes showed higher affinity to this herbicide than to structurally similar compounds (simazine, prometryn, and metribuzin). The binding capacity of MIP membranes was found to be significantly higher than that observed previously for MIP systems. The high affinity, specificity, and binding capacity of MIP membranes, together with their straightforward and easy preparation, provide a good basis for their application in separation and purification, e.g., in membrane chromatography.