Volatile organic compounds (VOCs) and odors in cattle rumen gas were characterized using in vivo headspace sampling with solid phase microextraction (SPME) coupled with gas chromatography-mass spectrometer-olfactometry (GC-MS-O) analysis. A novel device allowing for headspace SPME (HS-SPME) sampling through the cannula was designed, refined, and used to collect rumen gas samples from steers. Carboxen/polydimethylsiloxane (PDMS) fiber (85 µm) was used in the SPME sampling. Fifty VOCs belonging to 10 chemical functional groups were identified in the rumen headspace. The identified VOCs had a wide range of molecular weight (MW) (34 to 184), boiling point (-63.3 to 292 °C), vapor pressure (1.05×10^-5 to 1.17×10 ²Pa), and water solubility (0.66 to 1×10⁶ mg/L). Twenty-two compounds have a published odor detection threshold (ODT) of less than 1 ppm. More than half of the identified compounds are reactive and have an estimated atmospheric lifetime of < 24 hr. The amounts of VFAs, sulfide compounds, phenolics, and skatole, and odor intensity of VFAs and sulfide compounds in the rumen gas were all higher after feeding than before feeding. These results indicate that rumen gases can be an important potential source of aerial emissions of reactive VOCs and odor. In vivo sampling via SPME coupled with GC-MS-O analysis can be a useful tool for qualitative characterization of rumen gases, digestion, and its relation to odor and VOC formation.