The inexpensive and disposable electrode based on a double-polymer-modified pencil lead is proposed for upper-division undergraduate instrumental laboratories to enable the highly sensitive detection of perchlorate. Students fabricate and utilize their own electrodes in the 3-4 h laboratory session to learn important concepts and methods of electrochemistry. The simple electrodes allow for the detection of perchlorate in tap water at concentrations below an interim health advisory level of 15 ppb ([approximately]150 nM) set by the U.S. Environmental Protection Agency. Specifically, a pencil lead is first modified with a conducting poly(3-octylthiophene) (POT) membrane through electropolymerization by cyclic voltammetry, followed by the dip coating of a plasticized poly(vinyl chloride) (PVC) membrane. The PVC/POT-modified electrode is operated in the stripping voltammetric mode to give a linear current response to 100-1000 nM perchlorate in tap water. This high sensitivity is due to the thermodynamically favorable preconcentration of relatively lipophilic perchlorate from the aqueous sample into the lipophilic PVC membrane, which is driven by the oxidation of the underlying POT membrane. Additionally, students can use a PVC/POT-modified pencil lead to detect other anions of interest and evaluate their lipophilicity, which affects their environmental toxicity and pharmaceutical activity. (Contains 3 figures.)