Dark brown (Ag2AgII)-Ag-I(SO3F)(4), known for over 30 years, is the first known example of four well-characterized mixed-valent (1+/2+) compounds of silver. It crystallizes in the monoclinic space group, P2(1)/c, with a = 5.33670(19) angstrom, b = 12.9486(4) angstrom, c = 19.5976(7) angstrom, beta = 100.6407(13)degrees, V = 1330.95(8) angstrom(3), Z = 4 and d(calcd). = 3.59 g cm(-3). Its chemical formula is best written as [Ag-2(I)(SO3F)](+)[Ag-II(SO3F)(3)](-). Ag-II centres form 1D chains linked through OSO bridges that result in pronounced antiferromagnetic coupling with T-N = 225 K and a superexchange coupling constant (J) of -7.5 meV per pair of coupled Ag-II cations. Ligand environments around Ag-I and Ag-II differ substantially, which suggests a genuine mixed- (i.e., localized) and not intermediate-valent (i.e., delocalized) character of the title compound. Indeed, electronic absorption is not observed up to 7500 cm(-1), so the intervalence charge-transfer transition across the electronic band gap must fall above 0.8 eV. The compound is stable up to a mere 75 degrees C, which marks the onset of its thermal decomposition to (AgSO3F)-S-I and the SO3F center dot radical. (Ag2AgII)-Ag-I(SO3F)(4) is, after AgSO4, the second-known 1D antiferromagnetic semiconducting oxa derivative of Ag-II.