, 
, and 
, which are equations for mole balance for each element or element valence state, activity of water, and ionic strength.
An initial gas-phase-composition calculation is needed if the composition of a gas phase is not defined explicitly, but rather, the composition of a fixed-volume gas phase is defined to be that which is in equilibrium with a specified solution composition. The equations for the initial gas-phase-composition calculation are the same as an initial solution calculation and are
,
, and
, which are equations for mole balance for each element or element valence state, activity of water, and ionic strength.
For initial gas-phase-composition calculations, the values of 
include only the aqueous concentrations and the corresponding mole-balance equations 
do not contain terms for the contribution of the gas components to the total element concentrations. The values calculated for all quantities related to the aqueous phase are the same as for the solution without the gas phase present. Once the distribution of species in the aqueous phase is determined, the partial pressures of all components in the gas phase can be calculated. The partial pressures and the specified fixed volume are used with the ideal gas law to calculate the moles of each component in the gas phase.
All equations for initial gas-phase-composition calculations are included as equality constraints in the solver. No equations are optimized and no inequality constraints are included.
An initial gas-phase-composition calculation is performed only if the gas phase is defined to have a constant volume and is defined to be initially in equilibrium with a specified solution. The distribution of species for this solution has already been calculated, either by an initial solution calculation or by a batch-reaction or transport calculation. Thus, the values of all master unknowns related to the aqueous phase are known and are used as initial estimates for the initial gas-phase-composition calculation.
For data input to PHREEQC, definition of the initial gas-phase-composition calculation is made with the GAS_PHASE data block (see "Description of Data Input").