The tungsten and molybdenum hexacarbonyls, M(CO)(6) (M = W, Mo), dissolve in fluorosulfonic acid, HSO(3)F, to generate the tungsten and molybdenum carbonyl cations, [M(CO)(4)](2+)(solv), which are transformed, by exposure to an NO atmosphere, into the tungsten and molybdenum carbonyldinitrosyl cations, [M(CO)(NO)(2)](2+)(solv), respectively. These complexes have been characterized by NMR ((183)W, (13)C, and (15)N), IR, and Raman spectroscopy, and they are the first well-characterized metal nitrosyl cations in strong acids or superacids although the spectroscopic techniques do not address the number or coordination mode of the solvent molecules. Their formation suggests that strong acids and superacids can hopefully be used to generate a number of metal nitrosyl cations as they have been successfully used for preparing a series of metal carbonyl cations.