Electron-probe microanalyses of natural krennerites and sylvanites, containing no minor elements, have supported previous estimates of silver content using a d-value curve obtained from synthetic samples. The correspondence for the silver content of natural calaverite is not well established. Examples of krennerite and calaverite containing minor elements have been found. Krennerite can incorporate minor Cu in its structure which permits the mineral to have a lower percentage silver than normal krennerite. Calaverite has been found to contain both Cu and Sb in minor amounts permitting the structure to exist with more silver than in the Au-Ag-Te system.