By means of a study of magnetoquantum oscillations in the differential capacitance, the authors have observed the thermalisation of the space charge stored dynamically in the quantum well of an asymmetric double-barrier resonant tunnelling heterostructure based on n-GaAs/(AlGa)As. Fourier analysis of the oscillations was used to monitor the charge build-up in both the emitter accumulation layer and in the well. The storage time of an electron in the well was found to be approximately=0.5 mu s. The resonant tunnelling is truly sequential rather than coherent.