An electrochemiluminescence (ECL) sensor having a Pt electrode coated with a tris(2,2＇-bipyridine)ruthenium (II)(Ru(II) complex)-modified chitosan responded to oxalic acid more strongly than to several other substrates. The ECL response intensity to oxalic acid was reproducible within 5% (RSD) on 10 repeated runs. A calibration curve for oxalic acid gave a straight line in the concentration range of 0.1 to 10 mM (correlation coefficient, 0.997) and the detection limit (S/N = 3) was estimated to be 3 x 10(-5) M. Interestingly, the response to trimethylamine was 39 times as weak as that obtained with a bare Pt electrode in an aqueous solution where the Ru(II) complex coexisted. The appearance of the high selectivity can be explained partly on the basis of the electrostatic repulsion of the chitosan membrane having a positive charge with trimethylamine, as being supported by cyclic voltammetry.