As part of the development of fission products processing, a pyrochemical treatment system has been proposed in order to separate actinides and lanthanides. Experimental investigations shows the efficiency of the chosen system which is LiF-CaF2/Zn-Mg but the separation factor between actinides and lanthanides is not high enough. Through theoretical investigations, different kind of way have been considered to increase the separation factor. The way regarding the control of the activity coefficients values in the pyrochemical system has been opened. Experiments have been carried out in this field and display the influence of these coefficients on the distribution results. The value of the separation factor between uranium and lanthanum is 70 in a diluted system and reaches 3300 in a concentrated media. It seems that it could be even better by improving the composition of the solvents.