We investigate the interaction and dephasing of excitons and negatively charged excitons (trions) in ZnSe/Zn0.9Mg0.1Se single quantum wells by intensity- and temperature-dependent spectrally resolved transient four-wave mixing. The studies show that the incoherent electrons which are involved in the formation of trions are optically excited from the GaAs substrate. The homogeneous linewidths of the exciton- and trion-transition increase with increasing captured electron density that saturates at high excitation intensities. With increasing temperature the electron density in the quantum well is reduced due to thermal activation and a decrease of the trion peak intensity and of the exciton and trion homogeneous linewidths is observed.