The evaporation from the silicon melt, during Czochralski process, is an important effect for Phosphorus and Antimony dopants. The evaporation rate gamma was deduced from the measured axial profile of the resistivity converted into the concentration. For the heavily doped crystals, the value of gamma is very similar for both P and Sb: in the order of 5.5x10(-5) cm/s (which is significantly lower than the previously reported evaporation rates). It was concluded that the rate-limiting step for the evaporation process is neither the evaporation reaction itself nor the impurity transport through the flowing gas, but rather the transport through the melt that strongly depends on the melt convection. For low Phosphorus concentration, the transport through the gas is severely slowed-down due to a change in the dominant gaseous species, from P2 to P-1 - and becomes a limiting step. The evaporation rate is decreased and, in fact, becomes negligible.