Composting is an alternative method of animal mortality disposal suitable for on-farm emergency containment of infectious diseases. Mortality composting can produce a complex variety of gases and some of them are known to be odorous. To date, relatively little is known about the makeup and temporal trends of organic gases and odors produced and emitted during composting processes. In this research, utilizing gas characterization for monitoring of the composting process was investigated. Emissions of volatile organic compounds (VOCs) and odors produced during composting of three carcass cover materials (corn stalks, oat straw and corn silage) were qualitatively studied at a laboratory scale set-up. Headspace samples were analyzed with multidimensional gas chromatography - mass spectrometry – olfactometry (MDGC-MS-O). Headspaces of decaying plant materials were tested using 85 µm Carboxen/polydimethylsiloxane (CAR/PDMS) SPME fiber. Aerobic and anaerobic conditions representing extremes of composting conditions were simulated to determine if composition of the gaseous byproducts can be used to evaluate aeration effectiveness. Volatile fatty acids (acetic, propanoic, isobutyic, butyric, isovaleric, valeric, hexanoic and heptanoic) were found as indicators of anaerobic decomposition of corn stalks and oat straw. The chemical makeup of gas and odor emissions was observed to decrease with compost age and was different for aerobic and anaerobic conditions. Chemical makeup and temporal trends in specific VOCs can be useful in non-invasive and indirect determination of the aeration status and completion of the composting process inside the biosecurity containment.