The effect of CaF2 on the viscosities of the MO-SiO2-xMnO-yCaF(2) melts (M[=Ca or Ba]O/SiO2 = 0.4-0.5, x = 10[+/- 1] or 40[+/- 2] mol%, y = 0-15 mol%) have been studied at high temperatures. At MnO = 40[+/- 2] mol%, the viscosity of Ba-Mn-silicate melts is lower than that of Ca-Mn-silicate melts because Mn2+ is a strong network modifier in the former system. However, the viscosities of both alkaline earth silicate melts with MnO = 10[+/- 1] mol% are not affected by cation type because MnO activity is too low to affect the viscosity of the melts. The CaF2 addition decreases the viscosities of the Ba-Mn-silicate melts with low-MnO contents, while it has less impact on the viscosities of the melts with high-MnO contents. The effect of CaF2 on the relationship between viscosity and the structure of the silicate melts is quantitatively analyzed using the micro-Raman spectra of quenched glass samples. The NBO/Si ratio of alkaline earth silicate melts, which indicates the degree of polymerization of the silicate networks, has a linear relationship with the activation energy for the viscous flow of the melts.