We investigate the nonlinear optical dynamics of excitons and trions in CdTe modulation-doped quantum wells by time and spectrally resolved pump-probe experiments. We find that the nonlinearities induced by excitons are different from those due to the presence of the trions. From a comparison between the exciton and trion behaviours, we show that, at low temperature, trions are formed from excitons in about 10 picoseconds and, as the temperature is increased, a thermal equilibrium is reached between exciton and trion populations within 5 picoseconds.