Humidity-controlled terahertz time-domain spectroscopy (THz-TDS) of perfluorinated sulfonic-acid (PFSA) membranes robustly quantifies properties such as water uptake (WU) and hygral swelling and shrinkage. With regard to hygral swelling and shrinkage, it was determined that a negligible thickness change can be expected from repeated dehydration and hydration cycles. Humidity-controlled THz-TDS compares well to dynamic vapor sorption (DVS) techniques with the added ability to extract WU values across a sheet of membrane, and can potentially probe bulk, free and bound water states more easily and accurately than Fourier transform infrared (FTIR) spectroscopy techniques.