To investigate the relationship between physicochemical properties and demulsifying ability, the influence of culture pH on physicochemical properties of a demulsifying strain of Alcatigenes sp. S-XJ-1 was studied. The demulsifying strain grew slowly in acidic conditions but grew well under alkaline conditions, indicating that S-XJ-1 was an alkaliphile. The optimal culture pH was observed at 10 for cultivation of Alcaligenes sp. S-XJ-1. Under optimal conditions, the cell surface hydrophobicity of the strain exhibited the highest levels of microbial adhesion to hydrocarbon at 78.4% and a water contact angle at 114.6 degrees. The cell surface hydrophobicity was increased owing to an increase in the relative content of proteins on the cell surface analyzed by X-ray photonelectron spectroscopy. The cell surface hydrophobicity showed a positive correlation with the demulsifying ability of the bacterial strain. It was inferred that higher cell surface hydrophobicity and a lower negative charge of the demulsifying strain contributed to the higher dispersion rate into the water-oil interface, which improved its demulsifying efficiency. (C) 2011 Elsevier Ltd. All rights reserved.