A new membrane ion-selective electrode (ISE) based on the use of the diclofenac-toluidine blue ion associate (IA) as an electrode substance (EAS) is developed. It is shown that the optimal concentration of IA in the PVC membrane is 5 – 11%. The effect of the plasticizer content on the linearity and steepness of the electrode function, as well as on the lower limit of diclofenac ion determination is studied. The best ratio of a plasticizer to PVC is (1.0 – 1.5):1. The content of the plasticizer is obviously related to the electrode lifetime which is mainly determined by the frequency of usage being 7.0 – 7.5 months on the average. The best sensor characteristics were obtained using the following composition: ion associate — 7%, dibutyl phthalate — 56%, PVC — 37%. The electrochemical sensor has a linear dynamic range of 5.0 Ч 10 –4 – 5.0 Ч 10 –2 M, electrode slope of 46.0 mV/pc with a lower detection limit of 3.2 Ч 10 –5 M, and response time <15 sec. The low-cost sensor is easy to use, exhibits fast response, wide range of the electrode function linearity, high selectivity to diclofenac and can be used for at least seven weeks. The new method of diclofenac determination is developed and tested on model solutions and pharmaceuticals.