A lithium-selective polymeric membrane has been immobilised on the surface of a semiconductor transducer. This light-address able potentiometric sensor (LAPS) with the induced lithium selectivity has been prepared with the aim at further development of a Li(+) assay in liquid media, including biological fluids. The sensor developed exhibits Nernstian response interval of 10(-5) to 10(-1) mol l(-1) with an ionic sensitivity of (58 +/- 1)mV/pLi. The detection limit was found to be (3 +/- 1) x 10(-6) mol l(-1) Li(+). The sensor has high selectivity for lithium over sodium and potassium ions with selectivity coefficients of K(Li/Na)(POT) = (7 +/- 2) x 10(-3) and K(Li/K)(POT) = (5 +/- 1) X 10(-3) respectively, and a response time of less than 30 s. (C) 2002 Elsevier Science B.V. All rights reserved.