A novel two-dimensional surface molecular imprinting method using water-in-oil emulsion photopolymerization on a microporous polypropylene substrate was developed and used to separate the bronchodilator, theophylline, from the mild stimulant, caffeine, both of similar chemical structure. Surface molecular recognition sites were generated that could discriminate between these two molecules with a concentration-dependent separation factor of 4.9 +/- 10.8 for theophylline over caffeine at 0.2 mM (1:1 v/v) solution mixture. Besides demonstrating its proof of concept, the attractive features of this new imprinting method are the following: fewer imprint molecules are needed, it can be used to imprint from aqueous environments, postcrushing of the solid matrix is not needed, recognition sites are all near or at the surface reducing mass transfer limitations, and imprinting on synthetic microporous membranes (with convection and diffusion) instead of beads (with only diffusion) speeds up the process.