Abstract A mixture of acetylene and xylene is used as the carbon source to directly synthesize carbon nanotube (CNT)/alumina (Al2O3) microsphere hybrid structures by CVD. The hybrid structures are modulated by varying the acetylene ratio and reactor temperature. The addition of acetylene can significantly improve the CNT growth rate on Al2O3 at temperatures lower than 600 °C. A tenfold improvement in CNT growth rate is obtained at 550 °C when only 2 vol.-% acetylene is added. Meanwhile, acetylene favors the formation of bundles containing high-density CNTs. The increase in the temperature promotes the formation of CNTs with large diameters and low number density. Mass spectrometry (MS) is used to investigate the decomposition of a mixture of acetylene/xylene at temperatures ranging from 450 to 700 °C, under the experimental conditions used. Acetylene decomposes with an increasing rate with temperature from 450 °C, however notable decomposition of xylene is not detected below 700 °C, which corresponds to the formation of the CNTs of varied diameters. This study demonstrates the possibility of large-scale production of desired nano/micrometer multifunctional CNT/Al2O3 hybrid fillers using a gas/liquid mixed carbon source.