The solubility of cinnabar (red HgS) has been measured in aqueous sulfide solutions under varying conditions: up to 2.5 molal HS (super -) , pH 3.5 to 7.8; 25 to 200 degrees C; and 4 to 140 atm. total pressure. Concentrations were determined by using neutron activation analysis. From these measurements plus those of Schwarzenbach and Widmer for metacinnabar (black HgS), the following constants were derived for ionic strengths (mu ) and temperature as indicated:HgS + 2H 2 S(aq) --> HgS(H 2 S) 2 (aq)K = 10 (super -4.25) (black HgS, mu = 1.0, 20 degrees C)K = 10 (super -4.31) (red HgS, mu = 0, 20 degrees C)K = 10 (super -3.0) (red HgS, mu = 0, 100 degrees C)delta H = + 8.3 Kcal. (red HgS, 25-100 degrees Based on an ionization constant for NaHS of 5.0 at 20 degrees C, we also find:HgS + HS (super -) + H 2 S(aq) --> Hg(HS) 3 (super -) K = 10 (super -3.50) (black HgS, mu = 1,0, 20 degrees C)K = 10 (super -3.59) (red HgS, mu = 1.0, 20 degrees C)HgS + 2HS (super -) --> HgS(HS) 2 (super =) K = 10 (super -3.51) (black HgS, mu = 1.0, 20 degrees C)K = 10 (super -3.60) (red HgS, mu = 1.0, 20 degrees C)and from the constant of Schwarzenbach and Widmer,HgS + S (super =) --> HgS 2 (super =) K = 10 (super +0.57) (black HgS, mu = 1.0, 20 degrees C),we find K = 10 (super +0.48) (red HgS, mu = 1.0, 20 degrees C)The pH boundaries between the regions of predominance of these complexes in equilibrium with metacinnabar at 20 degrees C, mu = 1.0, are:HgS(H 2 S) 2 , 6.13, Hg(HS) 3 (super -) , 6.89, HgS(HS) 2 (super =) , approximately 8.5, HgS 2 (super =) where only the last pH is dependent on sulfide concentration.The maximum measured solubilities were 950 mg./l at 25 degrees C and 4,300 mg./l at 200 degrees C. In general, the solubility between 25 and 200 degrees C in an isochemical, isochoric system increases 4-7 times in bisulfide solutions, and the increase in H 2 S solutions is about twice as great as in HS (super -) solutions.Transport of HgS or Hg to form mercury deposits, by bisulfide complexes or, in strongly alkaline solutions, by HgS 2 (super =) , is possible under geologically reasonable pressures at 100 degrees to 200 degrees C in neutral to weakly alkaline sulfide solutions. On the basis of present evidence, the relative geological importance of the bisulfide and sulfide complexes is uncertain.