Two polymer hosts have been used to prepare composite electrolytes, i.e. PEO (M-w = 1 x 10(6)) and a triblock copolymer [poly(propylene glycol-ethylene glycol-propylene glycol)] with M-w = 2000. The effect of the LiAl5O8 addition on the properties of electrolytes prepared with polyether-LiI was studied. The aluminate ceramic powder was prepared by coprecipitation and calcined at 1150 degreesC. The main goal of the present investigation is to compare the two classes of composite electrolytes with polyether-LiI with a composition fixed at n = molar ratio of [O]/[Li] = 20. LiAl5O8 powder was dispersed by ultrasound in the electrolyte matrix in a range of concentrations up to 20 wt%. Differential scanning calorimetry curves indicated that all materials are semi-crystalline, and the triblock composites show a smaller glass transition (T-g = - 28 degreesC at 20 wt% of ceramic) than that of PEO electrolytes ( - 16 degreesC at 20 wt% of ceramic). Thermomechanical analyses were performed using penetration mode and the composite electrolytes exhibited a significant increase in mechanical stability. The ionic conductivity of both systems (PEO and triblock-LiI-LiAl5O8) is 5 x 10(-4) S/cm at 80 degreesC. The triblock composite shows a conductivity of one order of magnitude higher than that of the PEO material near room temperature as a consequence of the decrease in crystallinity after a first heating. This result raises the interest in the development of composites based on a triblock polyether.