A study of the sticking coefficient of Mg vapour on in situ Ar and N2 plasma-treated polypropylene (PP) is presented. After exposure of the pretreated sample to a determined amount of Mg vapour, X-ray photoelectron spectroscopy (XPS) allows measurement of the adhered Mg amount at the polymer surface and the chemical nature of the interface. The sticking coefficient on an as-received sample is zero and is increased to several tenths depending on the pretreatment conditions, namely the nature and the pressure of the neutral gas, the treatment time, and the applied RF-bias. Relations between the plasma parameters, the XPS measured surface state before the metallization, and the sticking coefficient are investigated.