The electrical insulating properties under pressure of a number of halogenated methane compounds and sulphur hexafluoride are compared; sphere-sphere and point-sphere electrode configurations, and alternating, direct and impulse voltages are used. The influence of irradiation is also investigated. Of the gases studied, sulphur hexafluoride and difluorodichloromethane (known commercially as Arcton 6) have the most favourable properties for high-voltage insulation, and additional data are provided for the comparison of these two gaseous compounds. This includes (a) power-frequency breakdown-voltage characteristics of admixtures with nitrogen in uniform and divergent fields, (b) the comparison of the radio and power-frequency uniform-field breakdown voltages at atmospheric pressure, (c) the breakdown-voltage characteristics for a number of concentric-cylinder electrodes at various gas pressures with alternating, direct and impulse voltage, (d) the surface flashover-voltage/gas-pressure characteristics of solid insulation subjected to various forms of tangential stress.Data pertaining to electronegative gases are compared with corresponding data for nitrogen under the same conditions and with transformer oil at atmospheric pressure.Chemical and physical properties, such as decomposition in the presence of electrical discharges and vapour pressure characteristics, and economic factors are considered in assessing the suitability of sulphur hexafluoride and difluorodichloromethane as insulants for high-voltage equipments.Factors contributing to the high electric strength of some electronegative gases are briefly discussed.