The anation and decomposition reaction of trans[CrF(H2O)(aa′)2]S2O6 [aa′ = ethylenediamine (en); 1,3-diaminepropane (tmd)] have been studied by means of ATD, TG and DSC techniques. The first and only perfectly defined step is the anation reaction forming the species cis[CrF(S2O6)(aa′)2]. By means of DSC techniques, the enthalpies of reaction and the activation energies of both reactions have been estimated. The electronic spectra of the resultant products suggest cis configuration. A second step, preceding total decomposition of the complex, involves cis[CrF(S2O6)(aa′)2] → cis[CrF(SO4)(aa′)2] + SO2 which is contaminated by decomposition reactions. © 1981.