Various models of the dynamic z-pinch are discussed and photographic evidence from experiments using hydrogen and deuterium discharges in a straight tube is given in support of a shock model which allows for energy spent in ionization and dissociation. Effectively, the ionization and dissociation reduce γ from 5/3 to the region of 1.2. The temperature of the shocked gas is calculated from the measured shock velocity, and a curve of temperature against initial gas pressure is given. This temperature varies between 0.5 and 25 eV for pressures of 10,000 μ to 100 μ which corresponds to 10 to 30 per cent of the directed radial energy of the ions. Thus the pinch, during the implosion phase, consists of a rapidly imploding skin of low energy plasma.