A new membrane ion selective electrode sensitive to selenite ion has been developed. The electrode consisted of 1,2-phenylenediamine selen complex PIS (piaselenol) as the active material, PVC or SR (silicon rubber) as membrane matrix and DBF (dibutylphtalate) as plasticizer. This electrode showed linear response for selenite ion in the 10(-5)-10-M-1 concentration range. The slope of the linear portion was 21 mV/10-fold change in selenite concentration. The effect of membrane composition and membrane thickness on electrode response was studied and the electrode which contains 2% PIS, 49% PVC and 49% DBF was found to be the most sensitive one to selenite. The slope of the electrode did not change for 2 months and the pH change did not affect the response of the electrode in pH range of 3-9. The interferences of SO42-, SO32-, S2-, HPO4(2-), CI-, Br-, and I- are investigated and while no interference was observed for SO42-, SO32-, S2- and I-, a very small interference was observed for CI- and Br-. The selenium present in anodic slime is determined using this electrode. (C) 1999 Elsevier Science B.V. All rights reserved.