The viscosity of some borosilicate glasses increases with heat‐treatment time for phase separation. The viscosity increase in the initial stage of phase separation is attributed to the composition change of the separated phases. The origin of the viscosity increase in the later stage of phase separation is the subject of controversy. Namely, the viscosity increase has been attributed to two different mechanisms: (1) environmental relaxation and (2) the increase of silica content in the high‐viscosity silica‐rich continuous phase. Analyses are presented to show that the latter model is consistent with the experimental