ThermoML is an XML-based approach for storage and exchange of experimental and critically evaluated thermophysical and thermochemical property data. The basic principles, scope, and description of all structural elements of ThermoML are discussed. ThermoML covers essentially all experimentally determined thermodynamic and transport property data (more than 120 properties) for pure compounds, multicomponent mixtures, and chemical reactions (including change-of-state and equilibrium). The primary focus at present is molecular compounds. Although the focus of ThermoML is properties determined by direct experimental measurement, ThermoML does cover key derived property data such as azeotropic properties, Henry's Law constants, virial coefficients (for pure compounds and mixtures), activities and activity coefficients, fugacities and fugacity coefficients, and standard properties derived from high-precision adiabatic heat-capacity calorimetry. The role of ThermoML in global data submission and dissemination is discussed with particular emphasis on the new cooperation in data processing between the Journal of Chemical and Engineering Data and the Thermodynamics Research Center (TRC) at the National Institute of Standards and Technology. The text of several data files illustrating the ThermoML format for pure compounds, mixtures, and chemical reactions, as well as the complete ThermoML schema text, is provided as Supporting Information. Some important issues related to characterization of thermodynamic data are beyond the scope of this paper (uncertainty specification) or are considered in generic terms only (critically evaluated data). These issues will be considered in subsequent papers in this series.