This research has been conducted to investigate the effect of inlet subcooling on two-phase flow instabilities in a horizontal pipe system with augmented surfaces. Five different inlet temperatures are used to study the effect of inlet subcooling for five different heat transfer surface configurations. All experiments are carried out at constant heat input, system pressure and exit restriction. The effect of inlet subcooling on the steady-state characteristics and two-phase flow instabilities are studied for each configuration. The boundaries for the appearance of pressure-drop-type and density-wave-type instabilities are found and the effect of the inlet subcooling on these oscillations is studied for each configuration. Copyright (C) 2002 John Wiley Sons, Ltd.