Thermal degradation of modern and ancient linen fabrics was investigated by DSC and FTIR analysis in order to find a possible correlation between thermal behaviour and the age of linen, A similar pattern to that for cotton degradation was found, and the essential influence of impurities occurring naturally or occasionally was found. Moreover, in the case of old linen fabrics, a cleaning treatment can induce chemical degradation, which is favoured by fibre-weakening, as confirmed by SEM analysis. Although a possible ageing effect seems to be hidden by interference sources, thermal data can provide very helpful information about the status of an ancient linen fabric.