Pulse chemical vapour deposition and infiltration is a promising technique for densifying porous substrates. By cycling the reactor pressure, the in-depth depletion of the gas phase is limited. An experimental approach of the deposition process is presented : for determining the influence of the residence time, temperature, pressure and nature of the precursor on the gas phase composition and the deposition thicknesses at the external surface and within the pores of the substrate. Two types of mechanism are involved, governed by the maturation of the gas phase during the pulse. For low pressures, low temperatures and stable precursors such as CH4, the gas maturation is very limited, the deposition mainly occurs by slow decomposition of small molecules and good infiltration can be obtained. Conversely, for high pressures, high temperatures and more reactive precursors such as C3H8, a high maturation of the gas phase results in (i) rapid condensation of stacks of small carbon layers through heavy aromatic species and (ii) difficult infiltration of porous substrates.