Electrochemical measurements involving stabilized zirconia as a solid electrolyte were conducted in order to determine the activities of phosphorus in liquid {Co-P) alloys in equilibrium with solid cobalt at temperatures between 1573 and 1693K. By using the present data for the two-phase region of Co(s) + [Co-P}(1), the free energy for the solution of diatomic gaseous phosphorus in pure liquid cobalt at 1 wt% solution was derived as: (1/2)P-2(g) = P(1 wt% in Co) DeltaG(0)/J . mol(-1) = (-260,000+/-22,000)+(29.3+/-3.5)(T/K) (C) Munksgaard, 2002.