A method and a system for determining the density of an insulating gas in electrical apparatus in the vicinity of parts carrying electrical current, the method comprising the following steps: a) a reference temperature is measured outside the apparatus and in the vicinity thereof; b) the current passing through the apparatus is measured, and the temperature rise of the gas above the reference temperature is determined on the basis of gas temperature rise values as a function of current values and of various reference temperatures, said temperature rise values having been previously determined by testing or by a mathematical model; c) the gas temperature is computed by adding the reference temperature and the temperature rise; d) the gas pressure inside the apparatus is measured; and e) the density .rho. of the gas is computed on the basis of equations of state of the gas .rho.=F(T,P) which equations are tabulated data.