Heat transfer coefficients in horizontal flow boiling of the wide-boiling binary mixture of C2F6/C2H2F4 and of the ternary mixture C2F6/SF6/C2H2F4 are measured. The experiments are carried out with the condition of uniform inner wall temperature at the tube perimeter. Results of the two boiling regions - forced convective evaporation and nucleate boiling -are obtained. In the forced convective evaporation region the results are predicted best using a pure fluid correlation which takes mixture properties into account. In the nucleate flow boiling region a degradation of the heat transfer coefficients occurs. Provided that a certain heat flux limit is not exceeded, the so called boundary heat flux, the degradation has to be predicted with a suitable mass transfer controlled model, based on the ideal heat transfer coefficient. In the region of high heat fluxes an "effective" heat transfer coefficient, based on local total vaporization instead of the mass transfer controlled model, is more suitable than the previously cited ideal heat transfer coefficient.