The influence of blending with polyetherimide (PEI) and doping with HCl and H3PO4 on the properties of sulfonated polyether ether ketone (SPEEK) was studied. Blending with PEI first results in an increase and then in a decrease in membrane swelling at PEI concentrations greater than 5%. The electrical conductivity of blend membranes follows the same trend. Doping with the acids enhances the conductivity several times, and the effect of doping with HCl is more significant. PEI forms spherical particles dispersed in the SPEEK matrix and, at the same time, partially dissolves in SPEEK, which reduces the swelling of the matrix at higher PEI concentrations. The increase in the membrane capacity to absorb water at small PEI contents is due to the formation of new water adsorption sites along the interface between the particles and the matrix. A modified effective medium model yielded calculated results in good agreement with the measured conductivity values, when the experimental absorption data were used in the simulation.