Thermal decompositions of the trans-[Co(en)2Cl2]Cl and cis-[Co(en)2Cl2]Cl complex isomers have been studied using the TG–FTIR and TG–DSC–MS techniques. The measurements were carried out in argon atmosphere over the temperature range 293–873 K. The influence of the configurational geometry on the pathways of thermal transformations was discussed. Furthermore, kinetic studies on the reaction of the cis–trans isomerization of the [Co(en)2Cl2]Cl complex salt in methanolic solutions were performed using UV–Vis spectrophotometry at five temperatures in the 293–313 K temperature range. The observed rate constants were computed using the “Glint” program based on the global analysis. It has been found that the cis–trans isomerization of [Co(en)2Cl2]Cl is a first order reaction. Furthermore, the cis/trans [Co(en)2Cl2]Cl isomerization proceeds according to the dissociative mechanism.