This thesis studies the stability and rheological characteristics of water / fuel oil emulsions. The knowledge of an emulsions' behavior under different conditions is important when it comes to the wide usage of fuel oil and the possibility of achieving higher energy efficiency when mixing fuel oil with water. The analyzed materials were pure fuel oil, an emulsion with 10 wt. % of water and an emulsion with 20 wt. % of water. The emulsions were obtained by cavitation mixing, without using surface active components. In order to determine the change of the emulsions' characteristics during time, samples were taken four times in periods of seven days. Moreover, because of the assumption of different water content in the emulsions at the top and at the bottom of the vessel, as a result of the water's higher density compared to fuel oil, the samples were taken from the top as well as from the bottom of the vessel. The water content and the emulsions' stability were measured by thermogravimetry, where the loss of water was followed. A high difference between the top and the bottom water content suggests an emulsion of weak stability. From the rheological characteristics, the viscosity and its change with the increase of the shear rate and temperature was measured. The shear rate was in the range from 1 s^-1 to 100 s^-1, and the temperature increased in the first set of experiments from 25 °C to 50 °C, and afterwards from 25 °C to 100 °C. It was determined that the viscosity of water / fuel oil emulsions decreases with an increase of temperature and shear rate, and increases with an increase of the water content. Also, the viscosity results indicate that the stability of emulsions decreases with an increase of water content.