Flow boiling heat transfer coefficients for Freon R11 and HCFC123 in a smooth copper tube with an inner diameter of 1.95 mm have been experimentally investigated. The parameter ranges examined are: heat fluxes from 5 to 200 kW m(-2); mass fluxes from 50 to 1800 kg m(-2) s(-1); vapour quality from 0 to 0.9; system pressures from 200 to 500 kPa; and experimental heat transfer coefficients from 1 to 18 kW m(-2) K-1. It was found that the heat transfer coefficients are a strong function of the heat flux and the system pressure, while the effects of mass flux and vapour quality are very small in the range examined. This suggests that the heat transfer is mainly via nucleate boiling. The present experimental data were compared with some existing correlations and recommendations on their use is made.