When the addition of the differential amount of component i dniσ or dnis is effected at constant pressure p, the differential molar enthalpy of adsorption, Δ a H i σ or Δ a H i s also called the isosteric enthalpy of adsorption (q st) is defined as: \[\Delta _{a}H_{i}^{\sigma} = -q^{\text{st},\sigma} = U_{i}^{\sigma} - H_{i}^{\text{g}}\] \[\Delta _{a}H_{i}^{\text{s}} = -q^{\text{st},\sigma} = H_{i}^{\sigma} - H_{i}^{\text{g}}\] where His = (∂Hs/∂nis)T,p,m,njs and Hi^g is the partial molar enthalpy of component i in the gas phase, i.e. (∂Hg/∂nig)T,p,nig