ISO 14912:2003 defines the following quantities commonly used to express the composition of gas mixtures: mole fraction, mass fraction and volume fraction, as well as mole concentration, mass concentration and volume concentration. Relating to these quantities of composition, ISO 14912:2003 provides methods for the conversion between different quantities and the conversion between different state conditions. Conversion between different quantities means calculating the numerical value of an analyte content in terms of one of the quantities listed above from the numerical value of the same analyte content, at the same pressure and temperature of the gas mixture, given in terms of another of these quantities. Conversion between different state conditions means calculating the numerical value of an analyte content, in terms of one of the quantities listed above, under one set of state conditions from the numerical value of the same quantity under another set of state conditions, i.e. pressure and temperature, of the gas mixture. Gas mixture composition can be converted simultaneously between different quantities of composition and different state conditions by combination of the two types of conversion. ISO 14912:2003 is only applicable to homogeneous and stable gas mixtures. Therefore any state conditions (pressure and temperature) considered need to be well outside from the condensation region of the gas mixture and that of each of the specified analytes.