The physical phenomena responsible for the developing process during penetrant testing using liquid phase developers are analysed in this paper. It is shown that the developers's liquid phase intensively affects the developing process. The main physical parameters of the liquid phase that determine liquid drawing out of a capillary channel, taking into account the channel geometry, are found. A relation is derived to calculate the degree of filling in the capillary channel when liquid phases interact. This relation allows one to compare the sensitivity of different penetrant systems used for penetrant testing, and it also permits one to correctly choose substances for technologies employing the consecutive contact of liquids with capillary-porous bodies. A new means is proposed to demonstrate how to determine the pressure of saturated vapours of complex composition liquids.