Metal oxide nanofibers are promising candidates for gas sensing applications due to their high surface area, pseudo one-dimensional structure and semi-conducting characteristics. The sensitivity of metal oxide nanofibers to different gases can be further maximized by controlling their morphology and microstructure. In this context, nickel oxide nanofibers were synthesized with controlled morphology and microstructure through the electrospinning technique using a solution composed of nickel acetate salt and poly(vinyl alcohol) polymer followed by calcination and annealing. These nanofibers were then employed as hydrogen and ammonia sensors. The diameter and crystallinity of the nanofibers increased, whereas the surface area decreased, with increasing ratio of salt to polymer in the solution. Nanofibers with intermediate diameter, crystallinity and surface area were found to undergo the largest resistance changes when exposed to hydrogen and ammonia at elevated temperatures. Furthermore, we found that appropriate annealing of these nickel oxide nanofibers led to full crystallinity, with a drastic increase in crystallite size and several fold decrease in surface area. These fully crystalline nanofibers possessed much lower sensitivity to hydrogen and ammonia but faster recovery as compared to their semicrystalline counterparts. In addition to surface area variations, different dominant sensing mechanisms are proposed to explain the observed differences in sensing behavior. (C) 2015 Elsevier B.V. All rights reserved.