We discuss time-resolved experiments that study photo-excited e-h pairs in GaAs quantum wells via their THz response. Resonant generation of excitons leads to characteristic THz spectra dominated by 1s-2p intra-excitonic transitions, which provide a direct density gauge. Fundamental differences between the intra-excitonic and free-carrier conductivity enable quantitative analysis of multicomponent e-h gases. We examine exciton ionization dynamics, where the exciton fraction approaches a quasi-equilibrium value in agreement with the Saha equation. A Saha model that takes into account thermal excitation of both heavy- and light-hole pairs is derived. Such experiments identify non-equilibrium phases of e-h gases and their underlying quasi-particle densities and dynamics.