Active clays are notable for their high-swelling and sealing properties, making them valuable in various geotechnical applications, including deep repositories for nuclear waste and landfills. These properties arise from water physically adsorbed to clay particles, having an ordered structure, and greater density. Existing hydro-mechanical constitutive models for water retention behaviour prediction, which distinguish between adsorption and capillary water, are based on theoretical formulations and lack experimental validation. This work proposes a novel experimental methodology for the quantitative assessment of water adsorbed in active clays via thermogravimetric analysis (TGA). Following the proposed methodology, this work quantifies the amount of water stored (capillary and adsorbed) in active clays under controlled boundary conditions. Results show that capillary and adsorbed water are present in all hydraulic states, with the amount of adsorbed water content varying between 33% and 68% for the investigated compaction of 1.3 Mg/m3. Existing adsorption isotherm models are tested for the first time against experimental data for granular bentonite. The outcomes reveal the need for quantifying the amount of adsorbed water for suitably implementing microstructural models into predictive frameworks.