The microorganisms Pseudomonas aeruginosa, Pseudomonas putida and Bacillus subtilis, Bacillus cereus, Bacillus licheniformis and Bacillus laterospor strains are used as biological agents in the biodegradation process of a model mixture of hydrocarbons, as well as in a mixture of emulsifiers. These consist of AT 7, Tween-80, L-10 or Lutensol GD 70. The examination of hydrocarbon biodegradation was carried out using aseptic solutions and water samples from the River Warta in Poland. Significant effectiveness of hydrocarbon biodegradation of the examined bacterial strains and their mixtures in both was observed after seven days of biodegradation. The effectiveness of biodegradation depends on the type of emulsifier used. Results of the biodegradation experiments indicate the important role of the natural emulsifiers which are produced by the Pseudomonas aeruginosa and Pseudomonas putida strains. (C) 2003 Published by Elsevier Ltd.