The soil contamination by hydrocarbons is a real environmental concern in many parts of the world due to growing economic dependence on petroleum, mostly fossil fuels are often subject to leaks and accidental spills. Studies able to understand the mechanisms of biodegradation of gasoline components in unsaturated soil are necessary because they help in making decisions regarding the management and control of the propagation process in the subsurface. In Brazil, ethanol is used as an oxygenate gasoline additive and some authors suggest that for saturated soils, being more degradable, it delays the degradation of other more toxic components of this fuel. The previous study by this research group to unsaturated soil obtained similar results to those already observed for saturated soil. However, the mechanism of degradation in the unsaturated zone have few studies. This study aims to analyze the degradation mechanisms of the contaminant in unsaturated blocks of sand and silt soil from the city of Duque de Caxias - RJ, subjected to a pulse of contaminant. Two blocks, one sandy (BA) and silty (BS), were contaminated by two percent solution of benzene, four percent toluene in heptane (BT); other two blocks, also sandy (EA) and silty (ES), underwent the same solution added to twenty percent ethanol (BTE). Analyzes were performed to monitor the overall activity of the degrading medium, available carbon, the microbiota metabolic profile, concentration of the contaminants were performed as well as measurements of the water content through the use of Time Domain Reflectometer (TDR). The initial microbial activity before the contamination, showed low values. Soon after contamination of the blocks, those subjected to ethanol showed no activity while subject to BT mixture showed an increase, contrary to expectations. Microbial activities ranged throughout the experiment which may indicate an adaptation of microbes to new environmental conditions. Based on the results of testing and monitoring conducted, we suggest that the main determinant for changing the activity factor was the composition of the soil. The contaminant did not exert the expected influence and seen in previous studies