Abstract: A comparison was made of the hygroscopicity, cell wall chemical composition and crystallinity of recently peeled poplar (Populus spp.) wood and wood of the same species subjected to repeated cycles (20, 60 and 80) of vacuum/pressure (85 kPa/600 kPa) and soaking in an autoclave followed by oven drying. The 15 and 35 °C sorption isotherms were obtained using the saturated salt method and fitted with the Guggenheim–Anderson–de Boer model. Chemical composition was determined and the infrared spectra and X-ray powder and 2D diffractograms were obtained to identify differences in the wood with and without cycles. The cycles caused a statistically significant decrease in equilibrium moisture content (EMC) between the wood without cycles and the wood with cycles, a statistically significant lower contribution by the monolayer as the number of cycles increased (in the 15 °C isotherm in adsorption without cycles from 8.12% EMC to 6.16% with 80 cycles, in desorption from 10.23 to 8.13%; in the 35 °C isotherm from 7.45 to 5.57% in adsorption and from 8.86 to 6.54% in desorption), a decrease in the area of the hysteresis loop with significant differences between the wood without cycles and the wood with cycles, a statistically significant decrease in the percentage of cell wall components (in cellulose and extractives, in lignin content between the wood without cycles and wood with 60 and 80 cycles, and in hemicellulose between the wood without cycles and the wood with 80 cycles), a statistically significant increase in crystallinity between the wood without cycles (CRI% 52.1%) and the wood with cycles (CRI% 81.60–92.50%), and reorganisation of the cell wall ultrastructure, as seen in the increased size of the cellulose crystal of the fraction oriented parallel to the fibre.