In metalorganic vapor phase epitaxy of β-Ga2O3 using triethylgallium (TEGa) and O2 as precursors and Ar as the carrier gas, the gases directly above the substrate were sampled and analyzed by time-of-flight mass spectrometry. TEGa was found to decompose at 400 °C–600 °C via β-hydrogen elimination reaction to generate gaseous Ga, hydrocarbons (C2H4, C2H2, C2H6), and H2. When β-Ga2O3 was grown at temperatures greater than 1000 °C and with input VI/III ratios greater than 100, the hydrocarbons and H2 were combusted and CO2 and H2O were generated. The C and H impurity concentrations measured by secondary-ion mass spectrometry in the β-Ga2O3(010) homoepitaxial layer grown under these conditions were less than their respective background levels. Thus, to grow β-Ga2O3 without C and H contamination, conditions that favor the complete combustion of hydrocarbons and H2 generated by the decomposition of TEGa should be used.