CO2 has gained renewed interest as refrigerant among natural fluids thanks to its low environmental impact and its good thermodynamic and transport properties. CO2 provides very high heat transfer coefficients during flow boiling at medium reduced pressures. In addition, trends with vapor quality are remarkably different from conventional refrigerants working at the same saturation temperature. In this paper, the effect of the reduced pressure and its combined effect to other operating parameters (mass flux and heat flux) on flow boiling are experimentally investigated in a circular, horizontal, smooth tube of 6.00 mm inner diameter over a wide range of variation (from 0.19 to 0.64) combining experiments for R410A at low/medium reduced pressure to those at medium reduced pressure for CO2. The analysis of flow regimes and local peripheral heat transfer coefficients allowed to define some generalized trends, independent on the fluid, finding their relationship with the reduced pressure and other operating parameters. In such a way the local heat transfer coefficient trends are motivated by the different weights of the peripheral heat transfer coefficients around the tube. The whole database of 927 data points was used to perform a statistical analysis to assess the best prediction method for flow boiling at each reduced pressure. (C) 2012 Elsevier Ltd. All rights reserved.