It is well known that the efficiency of silica-based erbium/ytterbium-doped fibre lasers and amplifiers can be greatly enhanced by a high level of phosphorus codoping due to an improved Yb to Er transfer efficiency. The manufacture of these types of rare earth (RE)-doped silica fibres with a high P concentration is principally carried out via modified chemical vapour deposition (MCVD) technology in combination with solution doping. The supply of RE ions for the P-doped active core of the fibre preform occurs via the liquid phase during the multi-stage preparation process. The incorporation of phosphorus into the silica matrix is determined and strongly influenced by the chemical equilibrium, evaporation by formation of gaseous phosphorus oxides, and diffusion during the process steps. The knowledge and understanding of these interaction processes are very important for the optimisation of the fabrication process for high power laser and amplifier fibres. Here, we report on a systematic investigation of phosphorus incorporation into the silica matrix during the MCVD process in combination with a solution doping technique. The P2O5-doped silica soot material of the individual steps was prepared with a wide range of different process parameters (gas concentration of the starting compounds POCl3 and SiCl4, pre-sintering temperatures, porosity). The samples were investigated concerning their porosity, morphological characterisation, and composition via scanning electron microscopy, Fourier transform infrared spectroscopy and energy dispersive x-ray spectroscopy.