The chemistry of fluorine incorporation in silica glass preparatory to preform fabrication by MCVD process was critically analysed on the basis of the thermodynamic properties of the constituent reagents. The theoretical findings were compared with the experimental results, pointing out the anomalies observed by different workers. The addition of phosphorus is found to have an insignificant role in modifying the fluorine incorporation level. The accepted model of dependence of fluorine content in the glass on the partial pressure of SiF4 to the power of 0.25 finds poor agreement with the experimental data at low fluorine dopant concentration.