Commercial $Victrex^{(R)}$ poly(ether ether ketone) was dissolved and sulfonated using sulfonic acid at an elevated temperature of $55-70^{circ}C$. Various techniques such as thermogravimetric analysis, dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), and small-angle X-ray scattering (SAXS) were employed to investigate the mechanical properties and morphology of sulfonated poly(ether ether ketone) (SPEEK) film. Sulfonation temperatures were shown to have a direct effect on the degree of sulfonation. The DMA study showed that well-developed ${alpha}$-relaxation peaks related to chain relaxation in the ionic regions shifted to higher temperatures as sulfonation degree increased. However, the ${eta}$- and ${gamma}$-relaxation peaks showed that the peaks related to chain relaxation in the crystalline regions shifted to lower temperatures as sulfonation degree increased. These DMA results suggest that the sulfonic acid groups act as very effective plasticizers for the intercrystalline regions. DSC thermograms showed that the glass transition shifted systematically to higher temperatures as the sulfonation degree increased. In the SAXS study, an ionic domain peak was observed and the peak position shifted to a slightly higher angle as sulfonation degree increased, implying that the number of ionic aggregates increased.