Biodegradation of a model mixture of hydrocarbon (dodecane and hexadecane, 1:1 w/w), as well as hydrocarbon emulsified by synthetic and natural emulsifiers, and their mixtures were analyzed in the pres-ence of Pseudomonas spp. and Bacillus spp. bacterial strains. Changes in surface tension were measured by the static and dynamic methods during biodegradation processes. During the biodegradation processes of hydrocarbon and emulsified hydrocarbon, various changes in surface tension were observed. In some systems, the initial decrease of surface tension after the addition of an emulsifier was greater after a few days of biodegradation. Decrease in the surface tension does not seem to be related to the degree of hydrocarbon biodegradation, both for the hydrocarbon and emulsified hydrocarbon systems.