A chemiluminescence one-shot sensor for hydrogen peroxide is described. It is prepared by immobilization of cobalt chloride and sodium lauryl sulphate in hydroxyethyl cellulose matrix cast on a microscope cover glass. Luminol, sodium phosphate and the sample are mixed before use and applied on the membrane by a micropipette. The calibration graph is linear in the range 20-1600mug/L, and the detection limit of the method (3sigma) is 9mug/L. A relative standard deviation of 4.5% was obtained for 100mug/L H(2)O(2) (n=11). The sensor has been applied successfully to the determination of hydrogen peroxide in rainwater.