The dissolution of amorphous SiO2 particles in CaO–Al2O3–SiO2 slags was investigated at 1450°C by high-temperature confocal scanning laser microscopy (HT-CSLM) and thermodynamic/kinetic analyses. The SiO2 particles used in this experimental study had a spherical form so that any rotation of the particle did not cause errors in the determination of the particle size during the dissolution. Moreover, a wide composition range of the slag could be chosen without forming any solid reaction layer which could distort the evaluation of the dissolution mechanism. The evolution of the diameter of the spherical SiO2 particle was measured by image analysis of pictures obtained from the HT-CSLM. It was found that the dissolution curve of the SiO2 particle (size as a function of time) exhibited either a parabolic-like curve or an S-shaped curve depending on the slag composition. The patterns were compared with a well-known shrinking core model (SCM), and it was shown that the SCM could not represent the dissolution behavior of the SiO2 particle observed in this study. It was experimentally found that the shape of the dissolution curves varies as a function of the slag composition. The curve exhibited a parabolic-like shape for low SiO2-containing slags and changed to an S-type shape with increasing SiO2 concentration in the slag. To elucidate the dissolution mechanism, a model based on approximations for the diffusion near the particle was proposed by modifying the previously available model [M. J. Whelan, Met. Sci. J., 3, 95–97 (1969)]. From the experimental data and the model calculations, the viscosity of the slag was shown to be the major factor affecting both dissolution rate and mechanism. Effective binary diffusion coefficients were estimated using the model and experimental data. Those were shown to be in the range of literature data.