Ultrafast femtosecond Coulomb explosion of charged homogeneous (Xe-n) and heterogeneous doped (HLArn) small and medium sized clusters (n<60) is studied resting on the picture of a vertical high-order multiphoton ionization from the ground state nuclear configuration. The final average atomic velocity (simulated at constant charge) increases with increasing the cluster size, and at constant cluster size increases linearly with the ion charge, in accord with the predictions of an analytical model. The linear dependence of the reciprocal explosion time on the charge is also in accord with the analytical prediction. From the energetics of the Coulomb explosion (reflecting a probable initial atomic distribution of the cluster size for small clusters); a nonvertical multiphoton ionization during the Coulomb explosion cannot be inferred. (C) 1997 American Institute of Physics. [S0021-9606(97)01641-3].