Ionic liquid/sulphonated poly(ether ether ketone) composite membranes were prepared using following ionic liquids: 1,2,3-trimethylimidazolium dimethylphosphate, 1-hexyl-3-methylimidazolium dimethylphosphate, 1-dodecyl-3- methylimidazolium dimethylphosphate. For each ionic liquid/polymer pair 4 composite membranes were prepared with 10 wt.%, 20 wt.%, 30 wt.%, 40 wt.% ionic liquid content. Membranes were analyzed using strain-stress testing at various elevated temperatures up to 150 °C. Impact of temperature, ionic liquid content in membrane and structure of ionic liquids on mechanical properties of composite membrane were evaluated. It was found that temperature and ionic liquid content had highest impact on mechanical properties of composite membranes.