IEC 61125:2018 describes a test method for evaluating the oxidation stability of insulating liquids in the delivered state under accelerated conditions regardless of whether or not antioxidant additives are present. The duration of the test can be different depending on the insulating liquid type and is defined in the corresponding standards (e.g. in IEC 60296, IEC 61099, IEC 62770). The method can be used for measuring the induction period, the test being continued until the volatile acidity significantly exceeds 0,10 mg KOH/g in the case of mineral oils. This value can be significantly higher in the case of ester liquids.Additional test methods such as those described in IEC TR 62036 based on differential scanning calorimetry can also be used as screening tests, but are out of the scope of this document.This second edition cancels and replaces the first edition published in 1992 and Amendment 1: 2004. This edition constitutes a technical revision.This edition includes the following significant technical changes with respect to the previous edition:a) the title has been modified to include insulating liquids different from mineral insulating oils (hydrocarbon);b) the method applies for insulating liquids in the delivered state;c) former Method C is now the main normative method;d) precision data of the main normative method has been updated concerning the dissipation factor;e) former Method A has been deleted;f) former Method B has been transferred to Annex B;g) a new method evaluating the thermo-oxidative behaviour of esters is included in Annex C.