Although the employment of n-p heterojunctions is among the most popular strategies to increase the performance of gas sensors, there have been a few systematic studies to determine the optimal composition in n-p heterojunctions. This paper reports the results of a systematic study of (n) xSnO(2)-(p)(1-x) Co3O4 composite nanofibers (NFs) for gas sensing applications. Composite NFs were synthesized by the electrospinning method followed by annealing at 600 degrees C. For gas sensing studies, several gases at optimal working temperature (350 degrees C) were tested. Depending on the nominal composition, the sensors showed either n-or p-type behavior as well as different responses to the target gases. Furthermore, for all gases tested, the 0.5SnO(2)-0.5CO(3)O(4) sensor (nominal composition) showed the best gas sensing characteristics. The underlying gas sensing mechanism was examined in detail. The highest response observed in the 0.5SnO(2)-0.5CO(3)O(4) NFs sensor was primarily attributed to the major role of the p-Co3O4 nanograins as electron reservoir. In addition, the possible substitution of Co+2/Co+3 in Sn+4 sites, the catalytic effect of Co3O4 and generation of defects were likely to be the secondary reasons. This highlights the importance of the optimal composition for achieving the maximum gas-sensing performance in n-p composite NFs. (C) 2017 Elsevier B.V. All rights reserved.