Hydrogen peroxide (H 2 O 2 ) 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 ( δ 2 H, 230‰) and oxygen ( δ 18 O, 24‰) isotope values of the H 2 O 2 solutions was large, reflecting the broad isotopic range of dilution waters. This resulted in predictable linear relationships of δ 2 H and δ 18 O values of H 2 O 2 solutions that were near parallel to the Meteoric Water Line (MWL), offset by the concentration of H 2 O 2 in the solution. By grade, dilute (3 to 35%) H 2 O 2 solutions were not statistically different in slope. Although the δ 2 H values of manufactured H 2 O 2 could be different from those of water, rapid H 2 O 2 –H 2 O exchange of H atoms eliminated any distinct isotope signal. We developed a method to measure the δ 18 O value of H 2 O 2 independent of dilution water by directly measuring O 2 gas generated from a catalase‐induced disproportionation reaction. We predicted that the δ 18 O values of H 2 O 2 would be similar to that of atmospheric oxygen (+23.5‰), the predominant source of oxygen in the most common H 2 O 2 manufacturing process ( median disproportionated δ 18 O = 23.8‰). The predictable H‐O relationships in H 2 O 2 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 H 2 O 2 reagents and peroxide‐based explosive products, which may assist law enforcement in criminal investigations. Copyright © 2011 John Wiley & Sons, Ltd.