An experimental study was conducted to investigate the effects of density and viscosity on zero net liquid flow (ZNLF) in vertical pipes. Predicting liquid holdup under ZNLF conditions is necessary in several types of petroleum industry operations. These include predicting bottomhole pressures in pumping oil wells and the design of compact gas-liquid cylindrical cyclone (GLCC(C))(1) separators. Models are proposed to predict flow pattern transitions under ZNLF conditions and comparisons are made with commonly used vertical flow pattern transition criteria. Data was collected using a 3-in.-diam, 14-ft-section of transparent vertical pipe. Several different fluids, of differing density and viscosity, were utilized with air flowing at approximately 25 psig. Results are presented showing the liquid holdup and the flow distribution coefficient (C-o) as a function of density, viscosity, and superficial gas velocity. [S0195-0738(00)00402-7].