The formation of volatile substances such as hydrogen, methane, ketones, and aldehydes during alumina production using the Bayer process is of interest because of their process and occupational safety and health implications. To better understand how these substances are generated in the Bayer process, the decomposition of a selection of low- and high-molecular-weight organic materials has been studied at 180 degrees C in a synthetic Bayer liquor consisting of 6 m NaOH. The main vapor-phase products were identified using gas chromatography with thermal conductivity or mass spectrometry detection. The results showed that caustic degradation of organic compounds commonly occurring in Bayer process liquors produces a range of substances known to be present in alumina refinery emissions. The main volatile products were H-2, CH4, and certain carbonyl compounds with low odor thresholds, such as 2-butanone and propanone. There was also some evidence for the production of volatile pentacyclic compounds.