A heterogeneous water-oil mixt. was heated up to boiling in a 5-dcm3 cylindrical vessel to study the structures and phase transitions in the system (heat flux d. 10-70 kW/m(2), oil vol. fraction 2-17%). A viscous Itherm-12 oil (d. 834.8 kg/m(3) viscosity 10.3 mPa s, surface tension 214.1 10(4) N/m at 100 degrees C) was used in the study. Sloshing of the interphase boundary, breaking of the oil phase and formation of oil droplets, disintegration of the oil droplets and formation of a water-oil emulsion, dissipation of oil droplets and formation of a stable emulsion were subsequently observed during the study. The heat transfer coeff. decreased with increasing time of the heating, heat flux intensity, as well as oil content in the mixt. and at the vessel walls.