This paper is aimed to an experimental study on the flow patterns formed by heavy crude oil (488 mPa.s, 925.5 kg/m3 at 20 °C) and water inside vertical and horizontal 1 in. pipes. The interfacial tension was 29 dynes/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 of the superficial velocities. In contrast with other studies, the annular flow pattern (‘core annular flow’) was observed in both horizontal and vertical test sections. In fact this flow pattern typically occurs 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 two criteria that favor its occurrence in pipes.