Preparation of proton exchange membranes usually involves the use of solvents with high boiling points. Depending on the drying condition, the amount of residual casting solvent in the membrane could vary and result in the significant difference in the membrane morphology and properties. However, such important factor has been surprisingly overlooked in the past research. Here we deliberately prepared a series of sulfonated poly(ether ether ketone) (SPEEK) membranes with controlled amount of residual casting solvent. After all the remaining solvents in the membranes were removed with the treatment of acid and water, the morphology and properties of these SPEEK membranes were characterized. With the increasing amount of residual casting solvent, the proton conductivity of water equilibrated SPEEK membranes increased until reaching a maximum value that was almost twice as high as that of SPEEK membranes with the minimum residual solvent. Further increase of residual solvent content resulted the decrease in the proton conductivity of the membranes. In comparison, the water uptake kept increasing with the increasing amount of residual solvent. Scanning electron microscopy (SEM) analysis suggested that there was larger aggregation of hydrophilic regions for the membranes with larger amount of residual solvent. Positron annihilation lifetime spectroscopy (PALS) results revealed that the free volume in the membrane increased with the increase of residual solvent content. Compared to the SPEEK membranes with the minimum residual solvent, 5 times increase in proton conductivity at 50% relative humidity was achieved by varying the amount of residual solvent in the membrane.