Microbial adhesion to the oil-water interface is an important parameter in biodegradation of hydrocarbons to enhance uptake and metabolism of compounds with very low aqueous solubility, but the mechanisms of adhesion are not well understood. Our approach was to study a range of compounds and mechanisms to promote the adhesion of a hydrophilic bacterium, Pseudomonas fluorescens strain LP6a, to an oil-water interface. The cationic surfactants cetylpyridinium chloride (CPC), poly-l-lysine and chlorhexidine gluconate (CHX) and the long chain alcohols 1-dodecanol and farnesol increased the adhesion of P. fluorescens LP6a to n-hexadecane from ca. 30 to 90% of suspended cells adhering. In contrast, adjusting the ionic strength of the suspending medium only increased the adhesion from about 8 to 30%. The alcohols, 1-dodecanol and farnesol, also caused a dramatic change in the oil-water contact angle of the cell surface, increasing it from 24 degrees to 104 degrees , whereas the cationic compounds had little effect. In contrast, cationic compounds changed the electrophoretic mobility of the bacteria, reducing the mean zeta potential from -23 to -7 mV in 0.01 M potassium phosphate buffer, but the alcohols, 1-dodecanol and farnesol, had no effect on zeta potential. Even though both types of compounds promoted cell adhesion to the n-hexadecane interface, the mechanisms were different. Alcohols acted through altering the cell surface hydrophobicity, whereas cationic surfactants changed the surface charge density.