Physical properties of hot dense matter at megabar pressures are considered. The new experimental results on pressure ionization of hot matter generated by multiple shock compression of hydrogen and noble gases are presented. The low‐frequency electrical conductivity of shock compressed hydrogen, helium and xenon plasmas was measured in the megabar range of pressures. To reduce effects of irreversible heating and to implement a quasi‐isentropic regime strongly compressed matter was generated by the method of multiple shock compression in planar and cylindrical geometries. As a result, plasma states at pressures of the megabar range were realized, where the electron concentration could be as highne∼ 2×1023cM−3, which may correspond to either a degenerate or a Boltzmann plasma characterized by a strong Coulomb and a strong inter‐atomic interaction. A sharp increase (by three to five orders of magnitude) in the electrical conductivity of a strongly nonideal plasma due to pressure ionization was recorded, and theoretical models were invoked to describe this increase. Opposite effect was observed for lithium compressed by multiple shock up to pressures ∼ 200 GPa, where electrical conductivity was sharply decreased as pressure increased. © 2004 American Institute of Physics