Hydrogen peroxide (H2O2) is a widely used oxidizer with many commercial applications; unfortunately, it also has terrorist-related uses. We analyzed 97 hydrogen peroxide solutions representing four grades purchased across the United States and in Mexico. As expected, the range of hydrogen (δ2H, 230‰) and oxygen (δ18O, 24‰) isotope values of the H2O2 solutions was large, reflecting the broad isotopic range of dilution waters. This resulted in predictable linear relationships of δ2H and δ18O values of H2O2 solutions that were near parallel to the Meteoric Water Line (MWL), offset by the concentration of H2O2 in the solution. By grade, dilute (3 to 35%) H2O2 solutions were not statistically different in slope. Although the δ2H values of manufactured H2O2 could be different from those of water, rapid H2O2–H2O exchange of H atoms eliminated any distinct isotope signal. We developed a method to measure the δ18O value of H2O2 independent of dilution water by directly measuring O2 gas generated from a catalase-induced disproportionation reaction. We predicted that the δ18O values of H2O2 would be similar to that of atmospheric oxygen (+23.5‰), the predominant source of oxygen in the most common H2O2 manufacturing process (median disproportionated δ18O = 23.8‰). The predictable H-O relationships in H2O2 solutions make it possible to distinguish commercial dilutions from clandestine concentration practices. Future applications of this work include synthesis studies that investigate the chemical link between H2O2 reagents and peroxide-based explosive products, which may assist law enforcement in criminal investigations. Copyright © 2011 John Wiley & Sons, Ltd.