The type of mantle convection strongly depends on different kinetics effects such as heat transfers across the transition zones, chemic al diffusion during phase transformation or stress relaxation occuring in response to volume changes. At a macroscopic scale, we model these effects through a single parameter which is a frequency-dependent compressiblity in the transition zones, in order to see its inf luence on postglacial rebound observables. A realistic axisymmetric and compressible Earth model based on PREM parameters and on a visc oelastic rheology is submitted to the last Laurentide ice-sheet glacial load (110 ky cycle). Because of the lack of data, a wide range of time characteristics of phase transformations is tested (10ky-10My). It shows that the viscosities of the upper and lower-mantle are not much affected by kinetics effects but that the pressure-field is strongly delayed in the transition zones during loading. This ind icates a specific rheological behaviour of the tansition zones depending on both their thickness and their time characteristics.