The structure and thermal behaviour of boehmites (AlOOH) having different crystallite size were investigated. At room temperature, the structural study shows a variation of the atomic distances in the smallest sized sample, attributed to a surface relaxation, The study of the thermal behaviour, between 20 and 600 degreesC, revealed two thermal phenomena before the boehmite decomposes to alumina. The first phenomenon is a linear thermal expansion which affects the two lattice directions a and b, and yields a slight contraction of the cell. in the c direction. The cohesion of the bonding is maintained by a deformation of the cell. The second phenomenon takes place about 70-100 degreesC under the boehmite decomposition temperature and is characterised by a further expansion of the lattice leading to dehydroxylation and collapse of the structure.