This work aims at studying the progressive swelling process of three polydimethylsiloxane covalent gels, synthetized at several volume fractions, νc ; two good solvents (chloroform and heptane) and a theta-solvent (bromocyclohexane) were used as swelling agents. i) The macroscopic swelling was found to obey a simple addition law of volumes of solvent and polymer. ii) The transverse magnetic relaxation process of protons linked to PDMS chains is shown to reflect a progressive swelling process of elementary chains occurring according to two well defined steps. The first one (1 qd) and an excluded volume effect arises ; this is fully observed on gels at equilibrium with a liquid solvent. Superposition properties are interpreted from a basic formula describing the transfer of localization of NMR properties from a local space-scale to a semi-local one.