Praseodymium (Pr)-modified zinc oxide (ZnO) nanofibers have been fabricated using an electrospinning-calcination method. These Pr-modified ZnO nanofibers present porous morphologies containing numerous ZnO nanocrystallites with average sizes that are much smaller than those found in pure ZnO nanofibers formed by the same procedures. Most Pr is identified at the surface of / interface between the nanocrystallites. In addition to the morphological modifications, addition of Pr is also shown to enhance the crystalline quality of the ZnO. Consequently, the Pr-modified ZnO nanofibers have a higher UV emission efficiency and exhibit a much-enhanced UV emission-based O-2 sensing performance than the pure ZnO nanofibers. By way of illustration, the Prmodified nanofibers show O-2 sensing responses of R= 39% at room temperature and R= 71% at 115 degrees C (cf. R= 19% and 52% with the pure ZnO nanofibers at these same operating temperatures). These results suggest that electrospun Pr-modified ZnO nanofibers hold real promise for high-performance optical gas sensing applications.