The position of minerals in hypogene veins has little apparent relation to mineral solubility. This problem is attacked by a detailed study of the Mississippi Valley type lead-zinc deposits, in which galena has behaved as if it were more soluble than sphalerite. Several possible explanations of this behavior are advanced, but the increase in galena solubility is attributed chiefly to the formation of chloride complexes. Experimental work shows that concentrated chloride solutions (2 molal) are necessary to explain the observed mineral relations. Evidence is presented to show that high concentrations may be a common feature of hypogene solutions.