The enthalpy of melting and the freezing point depression of European beech (Fagus sylvatica L.) wood modified with 0.8, 1.3, and 2.3 M 1,3-dimethylol-4,5-dihydroxy ethylene urea (DMDHEU) were determined at different levels of moisture content above the fibre saturation point by differential scanning calorimetry. The results permitted estimations of the amount of water bound to the cell wall, non-freezing water (NFW), and pore size distribution. The NFW of wood modified with DMDHEU, calculated on a dry wood basis, was not significantly lower than that of the control. The ratio of bound to total water present in the sample was higher in unmodified than in DMDHEU-modified samples. The proportion of pores with a diameter <= 30 nm was 70% of the total cell wall voids for wood modified with 2.3 M DMDHEU and 18% for unmodified wood. These results indicate that DMDHEU reduced the pore size of the samples by occupying the void space present in the cell wall.