Phosphine, a trace gas, is usually detected in aerobic digestion. To explore the key steps of gaseous phosphorus release during anaerobic digestion, a digestive medium was inoculated with anaerobic sludge and fertilizer to produce phosphine. The relationship between phosphine and digestive products, such as methane, hydrogen, carbon dioxide, and volatile fatty acids, was investigated. The experimental results indicated that methane and phosphine competed with each other for electron donors. Meanwhile, the concentration of phosphine reached approximately 110 mg/m(3) under methane production inhibition. Metabolite analysis indicated that fermentation of the butyric acid type favoured the production of phosphine, with a pH range of 6.3-7.0. The phosphine content was positively related to acetic acid and hydrogen, with correlation coefficients (R-2) of 0.98 and 0.88, respectively. Molecular biology indicated that Azotobacter, Dechloromonas, and Ruminococcaceae were associated with phosphine production. To the best of our knowledge, this study is the first to explore the key step of gaseous phosphorus release in anaerobic digestion, providing a theoretical reference for phosphorus resource utilisation. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.