Hollow sphere composites were synthesized by a template-free hydrothermal method from MoO₃and In₂O₃. The spheres have a typical size of 80050nm and were characterized by XRD, FESEM, TEM, XPS. Gas sensors based on samples with different Mo/In composite ratios were fabricated and their gas sensing properties were studied. The results show that a Mo:In ratio of 1:1 in the composite gives the highest response, typically at a working temperature of 250℃. The response increases to 38 when exposed to 100ppm acetone at 250℃. This is 13.6 times better than when using pure MoO₃. The sensor shows improved selectivity, response, repeatability and long-term stability. Typical features include a large specific surface area, and high levels of chemisorbed oxygen and defective oxygen sites. The N-N heterojunction theory was used to explain the improvement of gas sensing performance.