The potentiometric performance characteristics of microelectronically fabricated solid-state planar ethylmorphine sensors based on silicon/silicon nitride substrate and a mixture of ethylmorphine-tetraphenylborate and potassium tetrakis(p-chlorophenyl)borate as ion exchangers in a plasticized PVC matrix are described. Sensors incorporating dibutylsebacate and dioctylphthalate plasticizers display linear and stable near-Nernstian responses for ethylmorphine concentrations over the range 1x10−2- 4.5x10−5M and 1x10−2- 2.5x10−5M with calibration slopes of 53.1±0.8 and 57.1±0.8 mV/decade, respectively. The lower limit of detection is 1 μg/ml. The working pH range is 2.5-6:5 and the selectivity of the sensors for ethylmorphine is substantially higher with respect to a large number of opiate derivatives, amines, amides, amino acids and inorganic cations. The sensors are used for direct potentiometry and potentiotitrimetry of ethylmorphine in aqueous solutions and in pharmaceutical preparations. Results with an average recovery of 98.2% and a mean standard deviation of 1.2% are obtained.