A mixture of substances, in particular a biocide for combating human and animal parasites comprising a) an A component containing at least one type of alcohol of general formula R1-OH or R1-OH--R1', wherein R1 and/or R1' is an alkyl radical associated with a hydroxyl group which comprises from 1 to 12, preferably from 1 to 6, carbon atoms, b) a B component consisting of at least one type of carbonyl compound of formula R2-CO--R2', wherein R2 and/or R2' is an aryl-, alkyl- or cycloalkyl radical associated with a carbonyl group containing from 1 to 5, preferably from 1 to 2, carbon or hydrogen atoms, and c) at least one type of essential oil in the form of a C component consisting of an aromatic hydroxy compound of formula R3-0H, wherein R3 is a compound of at least one type of cyclic aryl radical which can be substituted up to two times and at least one organic compound R3', wherein R3' is an aryl-, alkyl- or cycloalkyl radical, wherein the components A and B are brought into reaction with hydrogen peroxide and the component C is subsequently mixed.