The chemisorption of sulfur on Ni(111) has been studied by scanning tunneling microscopy. At initial adsorption individual S atoms appear as protrusions 0.3-0.4 angstrom high, forming a local p(2X2) structure near step edges. The p(2X2) structure is found to be a simple S overlayer structure. After further H2S exposure the surface undergoes a reconstruction to a (5 square-root 3X2) structure, accompanied by a homogeneous nucleation of small islands resulting from an ejection of Ni atoms from terraces. The Ni density of 0.80+/-0.03 ML in the (5 square-root 3X2) structure has been determined by measuring the amount of Ni atoms used to build the islands.