Activation energies and reaction paths for diffusion and nucleation of mono- and divacancy defects in hexagonal boron nitride layers are theor. investigated. Migration paths are derived using the nudged elastic band method combined with d.-functional-based techniques. We find a different behavior for migration of single boron and nitrogen vacancies with the existence of intermediate metastable states along the migration paths. The temp. dependence of entropic and vibrational contributions to the free Gibbs energies is explicitly taken in account. A rich phase diagram for vacancy migration is then obtained. Boron vacancies are first thermally activated and can migrate to form more stable BN divacancies. At high temps., the divacancies can further be activated. In the contrary, nitrogen vacancy migration is energetically unfavorable within all the temp. range below the m.p. of h-BN. [on SciFinder(R)]