IInVI quantum well structures containing a 2DEG of low density have been investigated by means of polarized photoluminescence, photoluminescence excitation and refl ectivity in external magnetic fi elds up to 20 T. The spin splittings of the excition X and the negatively charged exciton X are measured as a function of the magnetic fi eld strength. The behavior of the magnetic-fi eld-induced polarization degree of the luminescence line related to X demonstrates the formation process of negatively charged excitons from excitons and free carriers polarized by the external magnetic fi eld. We have determined the binding energies of the trion formed either with the heavy-hole or the light-hole exciton. The optically detected magnetic resonance (ODMR) technique was for the fi rst time applied to study the optical transition processes in a nanosecond timescale. The electron ODMR was observed with the detection on either the direct exciton or the negatively charged exciton X. Further evidence for the interaction of excitons with the electrons of the two dimensional gas are demonstrated by a combined exciton-cyclotron resonance line observed in refl ectivity and luminescence excitation, shake-up processes observed in photoluminescence as well as inelastic and spin-dependent scattering processes. Eects resulting from the exciton-electron interaction in the presence of a two-dimensional electron gas (2DEG) of low density at neaB 1, where ne is the electron concentration and aB is the exciton Bohr radius, became a subject of intensive investigations very recently. This interest has been stimulated by the observation of a negatively charged exciton X in CdTe/(Cd,Zn)Te modulation-doped quantum well (QW) structures [1]. In this paper we review several new eects observed in structures where the exciton interacts with a 2DEG of low carrier density. In detail we discuss: the spin splitting and polarization dependence of X and X in high magnetic fi eld [2]; the combined exciton-cyclotron resonance [3]; the optically detected magnetic resonance (ODMR) on X [4]; the shake-up process [5,6] and the spin-dependent broadening of excitonic states [7].