We investigated modifications to carbon steel surfaces due to the adsorption of a biosurfactant derived from Pseudomonas fluorescens bacteria cells using contact angle measurements (CAM) and Xray photoelectron spectroscopy (XPS). After conditioning carbon steel in solutions with three different concentrations of biosurfactant molecules: 0.05, 0.3 and 1 gL(-1), the average thickness of the biosurfactant layer on the carbon steel specimens was 7.9 +/- 0.3, 12.1 +/- 0.5 and 16.4 +/- 0.7 angstrom, respectively. The biosurfactants changed the composition of both the Fe2+ and Fe3+ mixed-oxide layer and the outer layer, mostly composed of Fe3+ associated with magnetite. Contact angle measurements indicate decreased hydrophobic properties after the carbon steel was modified by biosurfactant. It was shown that the carbon steel surface free energy depends on the biosurfactant concentration, due to an acquisition of strong electron-donating properties. (C) 2015 Elsevier B.V. All rights reserved.