We have investigated the physical properties of proton conducting polymer membranes based on a protic ionic liquid (IL). Properties such as ionic conductivity, melting point of the polymer phase, and glass transition temperature of the liquid phase are studied as a function of IL/polymer ratio and temperature. We observe an increased thermomechanical stability of the membrane with increasing polymer content. However, there is a concomitant decrease in the conductivity with increasing polymer content. This decrease is larger than what can be expected from the dilution of the conducting IL by the insulating polymer matrix. The origin of this decrease can be caused both by the morphology of the membrane and by interactions between the polymer matrix and the ionic liquid. We find a change in the glass transition temperature and in the temperature dependence of the conductivity with increasing polymer content. Both effects can be related to the physical confinement of the IL in the polymer membrane.