A measurement technology for trace impurities in hydrogen chloride (HCI) was developed in relation to future ultra-large scale integration (ULSI). We developed an electrical conductivity measurement system for analyzing liquified HCl with an in-line high pressure cell. The detection limit of this measurement system is 1.0 X 10(-11) S/cm, and the external leak rate of the cell is below 1. 5 X 10(-11) Torr - liter/s. The apparent temperature coefficient of the electrical conductivity is negative in the range from - 12 to 20-degrees-C at constant pressure, which is caused by a decrease of the density of HCI with increasing temperature. The real temperature coefficient of the electrical conductivity is positive at constant density We observed differences in the electrical conductivity of HCI at different lapsed times after filling and between different gas sources. An electrical conductivity of 5.5 X 10(-10) S/cm was obtained by repeated liquifaction of HCl at a pressure of 41 kg/cm2 and 15-degrees-C.