Thermally and chemically stable, sulfonyl imide-based polymer blends have been prepared from sulfonimide poly(arylene ether sulfone) (SI-PAES) and sulfonimide Parmax-1200 (SI-Parmax-1200) using the solvent casting method. Initially, sulfonimide poly(arylene ether sulfone) (SI-PAES) polymers have typically been synthesized via direct polymerization of bis(4-chlorophenyl) sulfonyl imide (SI-DCDPS) and bis(4-fluorophenyl) sulfone (DFDPS) with bisphenol A (BPA). Subsequently, SI-Parmax-1200 has been synthesized via post-modification of the existing Parmax-1200 polymer followed by sulfonation and imidization. The SI-PAES/SI-Parmax-1200 blend membranes show high ion exchange capacity ranging from 1.65 to 1.97 meq/g, water uptake ranging from 22.8 to 65.4% and proton conductivity from 25.9 to 78.5 mS/cm. Markedly, the SI-PAES-40/SI-Parmax-1200 membrane (blended-40) exhibits the highest proton conductivity (78.5 mS/cm), which is almost similar to Nafion 117® (84.73 mS/cm). The thermogravimetric analysis (TGA) and Fenton's test confirm the excellent thermal and chemical stability of the synthetic polymer blends. Furthermore, the scanning electron microscopy (SEM) study shows a distinct phase separation at the hydrophobic/hydrophilic segments, which facilitate proton conduction throughout the ionic channel of the blend polymers. Therefore, the synthetic polymer blends represent an alternative to Nafion 117® as proton exchangers for fuel cells.