The dynamics of exciton formation and decay are investigated in GaAs-Al XGa 1−XAs (x<0.4) multiquantum well structures under low excitation conditions by transient photoluminescence spectroscopy. The time resolution (10 ps) is achieved by using the frequency up-conversion technique. At low temperature (T < 50 K), the luminescence is dominated by localized exciton formation and recombination processes: significant Stokes shifts from the exciton absorption line, long non-exponential rise time (ranging from 200 ps to 600 ps) and short radiative lifetime (from 200 ps to 400 ps) are observed. At higher temperature (T > 80 K), the luminescence is dominated by free exciton radiative recombination with shorter formation time and much larger exciton lifetime (from 3 ns to 6 ns). We suggest that the exciton may trap efficiently at low temperature on interface defects, the areal density of which can be evaluated in a simple manner.