As an effective alternative to simple binary oxide chemiresistors, polynary oxides with excellent tunability of the composition and hetero-interfaces are considered as promising material platforms for designing highly selective and sensitive gas sensors. In this study, ternary spinel CoCr 2 O 4 hollow spheres and CoCr 2 O 4 –Cr 2 O 3 mixed oxide hollow spheres were prepared via one-pot ultrasonic spray pyrolysis using solutions with different cation compositions ( i.e. , Cr/Co = 2, 3, and 4), and their gas-sensing characteristics were investigated. The pure CoCr 2 O 4 hollow spheres exhibited an unusually high response to 5 parts per million (ppm) of p -xylene (ratio of resistance to gas and air = 61.4), with negligible cross-responses to 5 ppm of ethanol, toluene, benzene, trimethylamine, ammonia, formaldehyde, and carbon monoxide. When CoCr 2 O 4 –Cr 2 O 3 hollow spheres with discrete Cr 2 O 3 nanoclusters were formed using a spray solution with a Cr/Co ratio of 3, the xylene response was enhanced to 144.1, which allows the sensitive and selective detection of sub-ppm level p -xylene. The unprecedentedly high xylene selectivity and response in the present study are explained by the gas-accessible hollow morphology, the unique catalytic activity of the ternary and mixed oxides, and the enhanced chemiresistivity due to the formation of a heterojunction between CoCr 2 O 4 and Cr 2 O 3 . The novel ternary oxide-based gas sensors with excellent xylene-sensing performance can be used in indoor air-monitoring applications.