Gel-based SPEs are promising candidates as polymer electrolytes in electrochemical devices such as Li-ion batteries. In particular, PMMA-based gel electrolytes have shown electrical conductivity at room temperature as high as 10(-3) Scm(-1).One of the most important drawbacks of these gels is their lack of long-term structural stability, which determines a reduction of the contact area with the electrode and causes drops in conductance. We have observed macroscopic volume reductions (> 5%) in gels made of PMMA, ethylene carbonate (EC), dimethyl carbonate (DMC) and LiN(CF3SO2)(2). The timescale of this phenomenon depends on the ratio EC/DMC.In this paper we investigate the structural modifications induced in the above mentioned system by annealing for several months at room temperature. We show that a crystalline phase grows in a matter of weeks with a kinetics controlled by the ratio EC/DMC and disappears for longer annealings.