The response of an ion-selective membrane electrode to 1-butyl-3-methylimidazolium (BMIm) and other ionic liquid cations is presented. The sensing membrane comprised the ionic liquid BMIm-PF6 in a plasticized PVC matrix. The electrode showed a fast, stable and Nernstian response over a wide concentration range. The partition coefficients of the imidazolium cations between water and the membrane were obtained. A relationship between these values and the toxicity of the corresponding imidazolium chlorides was found. The sensor was applied to the determination of BMIm in spiked water and to monitor the biosorption of BMIm by dry biomass. (C) 2010 Elsevier Ltd. All rights reserved.