Vertically coupled microring sensors with a chemically selective polymer membrane surface customization have been demonstrated and experimentally characterized for two different target analytes (toluene and xylene) for the first time. The E/Pco polymer membrane coating utilizes selective diffusion to identify the target class of analytes. This selective membrane coating thus enables a non specific index of refraction sensor such as the demonstrated optical microresonator to achieve a higher level of analyte selectivity. © 2007 IEEE.