NiC2O4 center dot 2H(2)O decomposition in air and in helium was studied using TG, DTA and MS and FTIR analyses. It was found that the dehydration phase of reaction is not complete. Remaining water occluded in the pores of nickel oxalate is not liberated until the decomposition of oxalate ions starts. The residual water takes part in reaction of the basic nickel carbonate formation. The basic nickel carbonate decomposition is a source of traces of nickel oxide stated in the final product of decomposition along with metallic nickel. It was found that the double peak endothermic DTA signal which accompanies decomposition of nickel oxalate results from changing of the microscopic mechanism of reaction. (C) 2007 Elsevier B.V All rights reserved.