An optical oxygen sensor based on an Eu-III complex/polystyrene (PS) composite nanofibrous membrane is prepared by electrospinning. The emission intensity of [Eu(TTA)(3)(phencarz)] (TTA=2-thenoyltrifluoroacetonate, phencarz =2-(N-ethyl-carbazolyl-4)imidazo[4,5-f] 1,10-phenanthroline) decreases with increasing oxygen concentration, and thus the (Eu(TTA)(3) (phencarz)]/PS composite nanofibrous membranes can be used as an optical oxygen-sensing material based on emission quenching caused by oxygen. Elemental analysis, UV/Vis absorption spectra, scanning electron microscopy (SEM), fluores-cence microscopy, luminescence-intensity quenching Stern-Volmer plots, and excited-state decay analysis are used to characterize the obtained oxygen-sensing materials. A high sensitivity (l(N2)/l(O2)) of 3.38 and short response and recovery times (t(down arrow)=-5.0, t(up arrow)=8.0 s) are obtained. These results are the best values reported for oxygen sensors based on Eu-III complexes. The high surface area-to-volume ratio and porous structure of the electrospun nanofibrous membranes are taken to be responsible for the outstanding performance.