Abstract: A potentiometric electrochemical sensor for detecting gaseous concentrations of arsenic oxides or arsenic-bearing species at elevated temperatures. The sensor is comprised of: an ionically conducting solid state membrane which may be made of silver zirconium arsenate, sodium beta-alumina or silver beta-alumina; a reference electrode which may be silver wire packed in silver powder; and a working electrode which may consist of a platinum mesh electrode or a silver wire exposed to the arsenic oxide-bearing gas. The probe is responsive to arsine [AsH.sub.3 ] concentrations in the range 5-1000 ppm in oxidizing gases, such as air, which have been raised to the temperature range 600.degree.-900.degree. C. The compound silver zirconium arsentate is also novel.