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We have investigated how ambient humidity can affect quantitative measurements of elastic properties on the nanoscale. Using an emerging technique called atomic force acoustic microscopy (AFAM), two samples were examined: a thin film of fluorosilicate glass and a section of borosilicate glass. When experimental results were analyzed using a simple model of the atomic force microscope cantilever dynamics, values of the tip–sample contact stiffness k* increased approximately linearly with relative humidity. The effect is believed to be due to the presence of a humidity-dependent layer of water on the sample. To account for this, the data analysis model was extended to include viscoelastic damping between the tip and the sample. A damping term proportional to the relative humidity was used. The revised values for k* showed virtually no dependence on humidity. Thus, the subsequent calculations of the indentation modulus M from k* yielded similar values regardless of measurement humidity. These results indicate that environmental conditions can influence quantitative nanoscale measurements of elastic properties, at least in some materials.