Co/Ag/Co trilayers were studied in order to confirm our previous results on the laser irradiation improvement of GMR ratio in immiscible Ag/Co multilayers (MLs). E-beam deposited samples were processed by XeCl excimer laser at the fluences F = 0.15, 0.2, 0.3 and 0.36 J/cm(2) with n = 1-100 pulses of 30 ns duration. The temperature versus time evolutions were computed for the first laser pulse. Samples were studied by grazing incidence XRD, RES and resistometry. From results it follows that grain boundary diffusion of Ag across Co layers going through even during short irradiation times, may be the explanation of GMR improvement via transformation of continuous ML into discontinuous one.