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We prepared sulfonated octaphenyl polyhedral silsesquioxane (SPOSS) with its structure intact by using sulfuric acid (H 2 SO 4) at 40 °C. We discussed the effects of sulfonation using either chlorosulfonic acid (ClSO 3 H) and H 2 SO 4 on SPOSS structure and its incorporation into crosslinked sulfonated polyphenylsulfone (CSPPSU) composite polymer electrolyte membranes (PEM). An in-depth structural characterization using 29Si NMR and FTIR spectroscopies, MALDI-TOF mass spectrometry, and TGA show that phenyl loss occurred in SPOSS prepared using ClSO 3 H, resulting in a low ion exchange capacity (IEC) of 1.7 meq/g and a sulfonation degree (DS) of 1.9. In contrast, the phenyl remained intact in the SPOSS prepared using H 2 SO 4. Well-dispersed 348-nm-SPOSS-H 2 SO 4 was found to have an IEC of 2.4 meq/g and DS of 3.1 The resultant hybrid PEM of CSPPSU/SPOSS using SPOSS-H 2 SO 4 had improved swelling, mechanical and chemical strength, and conductivity. 1 wt% SPOSS-H 2 SO 4 loading had the highest conductivity (13.7 mS/cm) at 120 °C with 40 % RH. This shows the importance of the SPOSS nanocomposite structure in dispersions in the SPPSU polymer matrix and its performance as PEM. We report on an improved direct sulfonation process at a milder condition but sulfonation yield was improved. [Display omitted] • Structure analysis of sulfonated OPOSS prepared by H 2 SO 4 and ClSO 3 H were performed. • OPOSS sulfonated by H 2 SO 4 had minimal structure damage compared to ClSO 3 H. • In-depth structural analyses were done using 29Si NMR, MALDI-TOF, FTIR and TGA. • The composite polymer electrolyte membrane was prepared with improved performance. • Increased proton conductivity and strength of 76 % and 24 %, reduced swelling by 11 %. [ABSTRACT FROM AUTHOR]