This part of ISO 4263 specifies a method for the determination of the ageing behaviour of hydraulic fluids of category HFC as defined in ISO 6743-4 (see [2] in the Bibliography) and specified in ISO 12922 (see [3] in the Bibliography]. The ageing is accelerated by the presence of oxygen, water and metal catalysts at elevated temperature, and the degradation of the fluid is followed by changes in pH value and insolubles content. Other parts of ISO 4263 specify similar procedures for the determination of the ageing behaviour of mineral oils and specified categories of fire-resistant fluids used in hydraulic and other applications. NOTE For the purposes of this part of ISO 4263, the term “% (m/m)” is used to represent the mass fraction of a material.