The vertical annular pipe flow of two immiscible liquids with very different viscosities (also known as core flow) provides an efficient and low cost method for producing heavy oils in vertical wells using water as a lubricant. This liquid-liquid flow pattern is becoming attractive in the current Brazilian deep water production scenario. Understanding interfacial phenomena present in this flow pattern is crucial for appropriate design of the production system. New experimental data on wavespeed, wave length, amplitude and wave profile of interfacial waves and holdup in heavy oil-water core flow (crude oil with 500 mPa s viscosity and 930 kg/m(3) density) inside a 2.84-cm-i.d. vertical glass tube at room temperature are presented and analyzed. The in-situ volumetric fraction of the oil is determined from the kinematic wave assumption and agrees very well with the measurements obtained via a proposed optical technique. (c) 2006 Elsevier B.V All rights reserved.5441732140148