The boiling of liquids in microchannels/microstructures is currently of great interest due to its very unusual phenomena and its many potential applications in a wide variety of advanced technologies. The thermodynamic aspects of phase transformations of liquids in microchannels was analyzed to further understand the boiling characteristics and to determine the conditions under-which a portion of such liquids is likely to undergo phase change. A nondimensional parameter and related criteria, that determine the phase transition in microchannels. were derived theoretically. The size of the microchannels results in dramatically high heat fluxes and superheats for liquid nucleation when the microchannel is sufficiently small. The effect that the liquid thermophysical properties have on-the nucleation is also described by the analysis (C) 1997 Elsevier Science Ltd.