Hydrothermal alteration is considered to increase the likelihood of dome or flank collapse by compromising stability. Understanding how such alteration influences rock properties, and providing independent metrics for alteration that can be used to estimate these parameters, is therefore important to better assess volcanic hazards and mitigate risk. We explore the possibility of using whole-rock δ18O and δD values and water contents, metrics that can potentially track alteration, to estimate the strength (compressive and tensile) and Young’s modulus (i.e. “stiffness”) of altered (acid-sulfate) volcanic rocks from La Soufrière de Guadeloupe (Eastern Caribbean). The δ18O values range from 5.8 to 13.2‰, δD values from –151 to –44‰, and water content from 0.3 to 5.1 wt%. We find that there is a good correlation between δ18O values and laboratory-measured strength and Young’s modulus, but that these parameters do not vary systematically with δD or water content (likely due to their pre-treatment at 200 °C). Empirical linear relationships that allow strength and Young’s modulus to be estimated using δ18O values are provided using our new data and published data for Merapi volcano (Indonesia). Our study highlights that δ18O values can be used to estimate the strength and Young’s modulus of volcanic rocks, and could therefore be used to provide parameters for volcano stability modelling. One advantage of this technique is that δ18O only requires a small amount of material, and can therefore provide rock property estimates in scenarios where material is limited, such as borehole cuttings or when sampling large blocks is impracticable.