In horizontal flow boiling conditions the heat transfer is affected by the saturation pressure, the composition of the mixture, the vapour fraction, the heat flux and the mass velocity. Furthermore, the heat transfer coefficients are different at different axial and circumferential positions on the heat transfer surface. In this work the calculation of averaged heat transfer coefficients is based on local (point) values. For the prediction of local heat transfer coefficients of pure substances a new correlation is proposed. Local heat transfer coefficients for two pure substances (SF6 and CCl2F2) and mixtures were measured for the condition of uniform wall temperature over the whole composition range and various values of the affecting parameters. In the nucleate flow boiling region a pronounced reduction of the local heat transfer coefficients occurs, which is dependent on the liquid composition, the heat flux and the saturation pressure. This reduction of nucleate flow boiling heat transfer coefficients of mixtures is predicted by the model of Schlunder. However, from the experimental results it can be concluded, that the reduction in vaporization heat transfer is smaller for high mass velocities. With a simple model for the evaluation of the wetting boundary circumferential averaged heat transfer coefficients can be computed.