The behavior of a two-phase thermosyphon, consisting of a microchannel evaporator plate and a condenser, is investigated to gain insight into the system limiting instability. A microchannel plate has been fabricated with 56 square channels that have a I x I mm cross section and a length of 115 mm. Experiments have been conducted for various condenser heights with the heat flux as the control variable. A step increase in heat flux is used to quantify the response of the system, including variations in mass flow rate, temperature, and pressure drop. Results show that small fluctuations about the steady state give rise to the instability for situations with a uniform heat load. A predictive model based on the momentum equation is introduced to estimate the onset of instability, and the threshold heat flux is predicted to within 10% uncertainty. (C) 2008 Elsevier Ltd. All rights reserved.