The amperometric oxygen sensors of the type, Ag\Ag6I4WO4\PbSnF4\ sensing electrode (a mixture of iron(II) phthalocyanine (FePc), PbSnF4 and a whisker material) have been fabricated and effects of the incorporated whiskers on response properties have been examined to design the optimum sensing electrode mixture. The selected whiskers were several 9Al(2)O(3) . 2B(2)O(3) and TiO2 whiskers different in length and conductivity (with or without conductive SnO2 coating film). Fast response comparable to that of galvanic-cell-type sensors was realized with the sensing electrode mixture designed on the basis of the following two points: (1) the sensing electrode should be a ternary mixture of whiskers, FePc and PbSnF4 with a volume ratio of 1:7:2; (2) the whiskers should have a length around 20 mu m, an aspect ratio of about 20 and a good electronic conductivity. The mean response time, 25 s, was to date the shortest of those of known solid-state electrochemical sensors at room temperature. The incorporation also greatly improved other response properties such as transient behavior, drifts and hysteresis. The roles of the incorporated conductive whiskers are to form the conductive whisker networks in the sensing electrode mixture and to make the reduction sites of oxygen molecules electrochemically uniform by localizing the sites along the networks.