During the past ten years, much attention has been focused on the using of microbial enhanced oil recovery (MEOR) processes by the oil industry. Very limited data has been published that reports the interfacial tension between crude oil and bacteria solutions and there is very limited laboratory work conducted on the phase behavior of oil-microbial systems. Microbial process is complicated and in order to apply the process successfully it is essential to perform an extensive laboratory work covering all areas related to the process mechanism. The MEOR bacteria transport through the water phase and congregate in pore spaces at oil-water interface where they metabolize a very small amount of the oil to produce various biochemical: surfactants, solvents, very week acids, polymers and gases (mainly CO2). These biochemicals decrease the interfacial tension between the oil and water, reduce oil viscosity, change the system wetability and in many cases improve the volumetric sweep efficiency by removing paraffin and scale blockage from pore throats. This project focused initially on the study of the interfacial tension (TFT) between curds obtained from four different UAE reservoirs (AH, UZ, SH and UAD) and thermophilic bacteria solutions. The bacteria were obtained from UAE local environment Several crude oil/water/bacteria fluid samples were prepared. For these samples the salinity ranged from 0-100,000 ppm, and the crude oil concentration ranged from 10% to 90%. All .systems were mixed very well and kept without any further agitation for one month. An optimum salinity was observed where a middle phase microemulsion was developed. Oil concentrations, keeping other variables constant, had no effect on the over all phase behavior of the studied systems.