A model of the quasi-Debye loss mechanism in SrTiO3 and Ba0.6Sr0.4TiO3 perovskite ferroelectrics at microwave frequencies is presented. The field dependence and the size of the field-induced quasi-Debye loss contribution in SrTiO3 and Ba0.6Sr0.4TiO3 systems have been evaluated in the simple approximation of the isotropic phonon spectra of these materials. Good agreement between experimental and simulated results for the field dependence of dielectric loss for SrTiO3 ferroelectric material is observed for the weak dc fields [tunability n=epsilon(0)/epsilon(E)much less than2]. A significant feature in the field dependence of dielectric loss at microwave frequencies, a pronounced maximum, is predicted for SrTiO3 at higher values of the bias field (ncongruent to2) whereas in Ba0.6Sr0.4TiO3 the field dependence of the loss is found to be monotonic. The influence of the quasi-Debye loss contribution on microwave performance of SrTiO3 and Ba0.6Sr0.4TiO3 ferroelectric materials is analyzed. It is concluded that the contribution of this mechanism is large enough to affect the performance of tunable microwave devices based on SrTiO3 ferroelectric materials, whereas in the case of Ba0.6Sr0.4TiO3 ferroelectrics the contribution of the quasi-Debye loss mechanism is found to be relatively small. (C) 2005 American Institute of Physics.