Equilibrium thermodynamic modeling has been used to study thermochemical processes in the inductively coupled plasma (ICP). A thermodynamic model for the central ICP channel has been developed, which takes into account all realistic initial plasma compositions and all possible components at the equilibrium. The accuracy of the model was confirmed by a good agreement between the experimental and calculated values for the electron concentrations in the ICP analytical zone. Degrees of ionization for 75 elements in the temperature range from 6000 to 9000 K were calculated at each 1000 K. A relation has been developed for describing the mass discrimination function of registered ions in a mass spectrometer with the ICP (ICP-MS). Using the experimental detection limits of elements in the ICP-MS and calculated partial pressures of their ions in the ICP, similar well-defined discrimination functions were obtained for many Ar ICP-MS spectrometers of different producers and for He ICP-MS. This confirms the accuracy and validity of using the equilibrium model in the central ICP channel and allows the adjustment of the calibration of ICP-MS spectrometers in multi-component semi-quantitative analysis. The series of calculations have shown significant matrix ion contributions in the total ion current in the ICP. This has been confirmed by the experimental data and indicates that the temperature in the central ICP channel is usually less than 7000 K. The new model can be used to determine the total ion current and average ion mass at the interface entrance. This is necessary for quantitatively analyzing the processes of ion loss during their transport in the ion beam.