The extraction coefficients for the actinyl ions extracted with tri-n-butyl phosphate (TBP) from 2.0M CH3 COOH, CH2 ClCOOH and CCl3 COOH in various temperatures have been measured. Distinct discontinuities of the lnD vs. 1/T plots were interpreted as a proof for the extraction mechanism changes. Results obtained were used to calculate the thermodynamic functions of the actinyl complexation in the aqueous phase and of the partition process. It is proposed that the inner-sphere complex formation increases in the order acetates < chloroacetates < trichloroacetates.