To improve the handlability and safety of a perchlorate solution used as a stabilizer for chlorine-containing resins.This invention provides a perchlorate solution of improved handlability and safety and a method for heat-stabilizing chlorine-containing resin compositions by adding (a) 0.004 to 10 parts by weight of a perchlorate solution containing 1 to 60% by weight of perchlorate, 5 to 50% by weight of water-soluble organic solvent and 20 to 94% by weight of water, and (b) 0.001 to 10 parts by weight of at least one silicate compound represented by the general formula (I) to 100 parts by weight of a chlorine-containing resin: M(O)a nSiO2mH2O   (I) in which M is at least one metal selected from alkaline earth metals and aluminum, a is 1 when M is an alkaline earth metal and 3/2 where M is aluminum, n is from 1 to 5, and m is any positive integer.