The response of a cyanide ion-selective electrode (ISE) in the presence of copper has been investigated. The interferences of metal ions, like copper, on the response of the cyanide ISE prevents its use for routine direct analysis of metal cyanide containing waste water. In this work an improved understanding of the chemistry at the electrode, and the effects of different cyanide and copper concentrations and matrix pH values on the electrode response, has been developed. Based on this insight, a semi-empirical approach is described to allow calibration of the ISE enabling the estimation of total cyanide. While not as robust as, for example, total cyanide distillation, it provides an inexpensive, rapid and convenient approach for more frequent and widespread screening of industrial waste waters.