The polar surfaces of ferroelectric LiNbO3 have been investigated by an ab initio thermodynamical approach. Basing on density functional theory total energy calculations, we discuss the relative stability of a series of candidate surface structures with varying stoichiometry and surface reconstruction in dependence on the chemical environment. We determine the equilibrium geometry for the positively and negatively polarised surfaces and then discuss the influence of different stabilising mechanisms on the preferred terminations. Positive and negative surfaces are found to have different structure, stoichiometry and ionisation energy. The positive surface is found to contain more oxygen than the negative surface at similar conditions. Different stabilisation mechanisms like stoichiometry modification and reconstruction contribute to stabilise the polar surfaces (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)