This paper deals with the problem of blind separation of under-determined or over-complete mixtures (i.e. more sources than sensors). At first a global scheme to separate under-determined mixtures is presented. Then a new approach based on time-frequency representations (TFR) is discussed. Finally, some experiments are conducted and some experimental results are given. KEY WORDS ICA, BSS, Time-Frequency domain, over-complete or under-determined mixtures