Measuring and understanding the swelling and softening behaviour of plant fibres under controlled humidity in relation with their moisture content are critical for their valorization in composites applications. Indeed, they directly influence the processing and in- service behaviour of biocomposites. The aim of this work is to characterize, under controlled hygro- and hydrothermal conditions (from 20% to 73% RH and in immersion), the moisture content, dimensional variations and mechanical behaviour of natural fibre bundles from various botanical origins with contrasting biochemical and structural characteristics. Automated laser scanning cross-sectional measurements and micro-tensile tests conducted either in a climate chamber or in an immersion cell allowed measuring cross-dimensional variations and tensile properties of the fibre bundles in relation with their moisture content determined by DVS and WRV. These analyses lead to determine the surface hygro- and hydroexpansion coefficients of the different natural fibre bundles in relation with the structural characteristics of the cell walls [1]. The softening effect of water on the cell walls and resulting loss of stiffness and strength of fibre bundles is also discussed in relation with water sorption and swelling [2]. These results constitute key data for the predictive modelling of “in- service” mechanical behaviour of biocomposites.