Abstract: The equilibrium product in Fe-Cu-Ce,Fe-P-Ce or Fe-Ti-Ce solution have ben Mentified to be Ce_2O_2S,but not the intermetallic compounds of Ce against Cu,P or Ti.The equilibrium eonstant of Ce_2O_2S=2[Ce]+2[O]+[S] and the interaction coefficients between solutes in Fe-base solution have been determined as follows: K_(Ce_2O_2S)=2.57×10~(-5) e_(Ce)~(Cu)=-0.49;e_(Ce)~P=1.77;e_(Ce)~(Ti)=-3.62 e_(Cu)~(Ce)=-0.22;e_P~(Ce)=0.39;e_(Ti)~(Ce)=-1.23 ε_(Ce)~(Cu)=-128.0;ε_(Ce)~P=-224.1;ε_(Ce)~(Ti)=-710.0 Ce decreases the activities of Cu and Ti in Fe-base solution and increases their solubifities,as well as increases the activity of P and decreases its solubility.

Key words: Fe-base solution, Ce, Cu, P, Ti, interaction, oxygen activity