In this chapter the experimental data of electrical conductivity of hydrothermal systems reported at temperatures above 473.15 K are summarized. The summary tables also show the range of temperature covered in these studies and the number of data measured in the range 373.15 < T/K < 473.15. Since the limit for high temperature measurements is arbitrary we have also included in the summary tables the data for several aqueous electrolytes studied at temperatures above 373.15 K, with upper temperature limit below 473.15 K. The systems included in this chapter include binary aqueous electrolytes and ternary aqueous system containing two electrolytes or an electrolyte and a non-electrolyte.