Interpenetrated networks are made by cross linking linear polymer chains of two different chemical nature. We consider the kinetics of the microphase separation that occurs when such cross linking is performed on a blend where composition fluctuations are present. We assume a simple exponential relaxation between the known initial and final states. The relaxation time however is assumed to be dependent on the spatial scale, and to diverge for large enough distances. We find that the scattered intensity in a radiation scattering experiment exhibits a minimum that varies slowly with time. This is interpreted as the very slow relaxation of the initial fluctuations that are present in the initial blend and are trapped by the cross links. (C) 1997 American Institute of Physics.