The paper presents a theoretical model for liquid-vapor interface instabilities for R134a condensation flow in tubes. The model is based on linear Kelvin-Helmholtz instability and capillary instability theories that neglect the effect of gravity and take into account the surface tension and shear stress effects. The normal mode method was used to analyze the system instability reactions to various perturbation wavelengths and physical conditions. Three modes were obtained with one giving the greatest instability wavelength for each condition. The cooling temperature had little effect on the greatest instability. The greatest instability wave length increased with decreasing R134a mass flux and quality, with jumps between two instability states. This instability state transition can be used to predict condensation flow regime transitions for intermittent flow. The dimensionless intermittent flow criterion is in good agreement with available experimental data in the literature. (C) 2017 Elsevier Ltd. All rights reserved.