Liquid-phase microextraction (LPME) in microfluidic devices involves extraction of acids or bases by diffusion in a pH gradient, from aqueous sample, through a supported liquid membrane (SLM), and into aqueous acceptor phase. The SLM is an integrated part of the device, and separates the donor (sample) and acceptor phase channels. In this work, the geometry of the donor and acceptor phase channels were studied and optimized. With optimal channel geometry (12 mm length, 2 mm width, 0.12 mm depth), metoprolol, haloperidol, nortriptyline, and loperamide were extracted from 900 μL urine and into 25 μL stagnant 10 mM HCl using a mixture of dihexyl ether and tributyl phosphate 1:1 v/v as the SLM. During 30 min of extraction, where the sample was pumped into the system at 30 μL min⁻¹ and the acceptor phase was stagnant, recoveries exceeded 75 % and enrichment factors up to 28 were obtained. Evaluation of the analytical performance supported the reliability of the device in combination with HPLC-UV detection. Implementation of LPME in microfluidic devices is expected to increase in the future and the current paper provides experimental support for the importance of the careful design of the donor and acceptor channels.