The ageing of high polymer fibre materials can be studied by determining the changes of mechanical strength and degree of polymerization. The relationship between the two variables is the same for degradation by photolysis and by heat. Under the influence of catalysts, ageing, i.e. degradation rates, may increase without actual change of the constants of the equation that describes the relationship between the changes of mechanical strength and degree of polymerization. One of the constants of the equation is a sort of material constant which determines simultaneously light stability and electrostatic behaviour of the pure material. 22 references. -- AATA