The suggested isomerism of some NiL4Br2 complexes (where L is benzimidazole) was investigated by thermoanalytical techniques coupled with an FT-IR system and supported by spectroscopic techniques. Starting from compound NiL4Br2. (acetone)2 two main routes of thermal decomposition were found, the first one corresponding to a family of tetragonally distorted octahedral compounds and the second corresponding to square planar complexes. Reflectance and IR spectra were obtained at room temperature. Analysis of the data supports the theory that in the family of tetragonally distorted octahedral compounds two main structures are possible : (i) a dimeric cationic structure where the axial position is occupied by three bromide ions, one acting as a bridge between two nickel(II) ions while the fourth bromide is the counter ion; and (ii) a monomeric neutral structure where two bromides are in both axial sites. The dimeric structure changes to the monomeric one by a temperature-induced anation reaction. Two chloroform solvate compounds were also prepared. Desolvation mechanisms are discussed, new procedures are suggested for obtaining the already known compounds, and some new compounds of the series have also been prepared. : (i) a dimeric cationic structure where the axial position is occupied by three bromide ions, one acting as a bridge between two nickel(II) ions while the fourth bromide is the counter ion; and (ii) a monomeric neutral structure where two bromides are in both axial sites. The dimeric structure changes to the monomeric one by a temperature-induced anation reaction. Two chloroform solvate compounds were also prepared. Desolvation mechanisms are discussed, new procedures are suggested for obtaining the already known compounds, and some new compounds of the series have also been prepared.