The metal chelates of 3-chloro-, 3-bromo- and 3-iodo-derivatives of lawsonemonoxime with La(III), Sm(III) and Dy(III) have been synthesised. The thermal, spectral and magnetic properties of these chelates have been investigated to study the effect of variation of the C-3-halo-substituent on the chelating ability of the lawsonemonoxime. The energy of activation E(a) and the order of reaction n for the decomposition stages have been calculated using the Coats-Redfern relation.