The objective of this study was to obtain a better understanding of the way in which the response of individual cellulose fibres to changes in relative humidity is relevant to the dimensional behaviour of paper and to the role of the fibres in composite materials, where dimensional instability is the main impediment to their wider use. The torsional response of individual pine tracheids to changes in relative humidity has been measured, and it is proposed that the observed angle of twist on drying a collapsed fibre is a function of the microfibril angle, the wall thickness, the fibre length and the fractional linear shrinkage across the microfibrils. A link between fibre twist and paper shrinkage is suggested. In thermomechanical pulps the temperature of defibration apparently affects the fibre twist in a way which is inversely related to the degree of fibrillation and fibre damage. The mechanical properties of wet fibre webs have been studied as a function of moisture content. The wet web strength and other properties of thermomechanical pulps depend on the fibrillation induced during refining which in turn depends on the relationship of the refining temperature to the lignin glass transition point.