This paper describes the response properties of an amperometric solid-state oxygen sensor of the type, Ag / Ag6I4WO4 / PbSnF4 / Sensing electrode, O-2, in which a mixture of Si-Ti-C-O fiber, iron(aU) phthalocyanine and PbSnF4 is used as a sensing electrode. A good linear relation was found between the short-circuit current and the square root of oxygen partial pressure. Addition of the conductive Si-Ti-C-O fiber reduced the mean 90% response time (t(90)) greatly to 25-30 s at room temperature. This is the shortest so far reported in ambient temperature solid-state oxygen sensors.