This paper is aimed to an experimental study on the flow patterns formed by heavy crude oil (initial viscosity and density 488 mPa s, 925.5 kg/m(3) at 20degreesC) and water inside vertical and horizontal 2.84-cm-i.d. pipes. The oil-water interfacial tension was 29 dyn/cm. Effort is concentrated into flow pattern characterization, which was visually defined. The similarities with gas-liquid flow patterns are explored and the results are expressed in flow maps. In contrast with other studies, the annular flow pattern ("core annular flow") was observed in both horizontal and vertical test sections. These flow pattern tends to occur in heavy oil-water flows at low water input fractions. Because of the practical importance of core flow in providing an effective means for heavy oil production and transportation, this paper discusses criteria that favor its occurrence in pipes.