The synthesis and comparative characterization of molecularly imprinted polymers (MIPs) in two different formats, as thin layers grafted to the entire surface of polypropylene microfiltration membranes and as conventional particles, are described. Imprinting with atrazine was performed by using itaconic acid and N,N'-methylene-bisacrylamide as functional and crosslinker monomers in methanol as the solvent. Polymn. had been initiated by UV irradn. of benzoin Et ether and driven to low monomer conversion for the thin-layer polymers and to high monomer conversion for the bulk materials. The binding performance of MIP composite membranes and of MIP particles packed into cartridges was evaluated in solid-phase extn. (SPE) expts. of atrazine and simazine from aq. solns. The SPE performance depended strongly on pH and buffer concn. Although an imprinting effect was obsd. for both formats, the specificity (MIP vs. Blank) and the selectivity (atrazin vs. simazin) were much higher for the thin-layer composite membranes than for the bulk polymer particles. In particular, the atrazin/simazin selectivity increased from 32% for the Blank to 78% for the MIP composite membranes. A major reason is the hindered accessibility of the internal pore structure of the particles, whereas the porous filtration membranes are much more compatible with the fast SPE protocol. Furthermore, based on pKa of the functional carboxylic acid groups-from potentiometric titrn. and polarity of the binding environment-from fluorescent probe anal., different properties of the imprinted binding sites can be postulated for the two MIP formats. However, the differences between MIP and Blank were much more pronounced for the thin-layer composite membranes. The hydrophobic surface of the polypropylene membrane appeared to be a major factor affecting the binding performance of thin-layer MIPs. The new porous composite membranes could be particularly useful as selective SPE materials in environmental, pharmaceutical, and anal. applications.