The dynamics of microscopic bubbles in low viscosity insulating liquids has been widely investigated. It has been shown that the bubble motion (growth, collapse, and rebounds) in such liquids is governed by inertial forces. In this paper, the results of an experimental study of the dynamics of microscopic bubbles in viscous liquids (μ≥3 mPl) are presented. It is shown that, according to the conditions (injected energy, liquid viscosity, and applied pressure), the bubble motion is greatly modified. For example, no bubble rebound is observed in the higher viscosity liquids (e.g., Napvis XD110, μ=83.5 mPl) and, for a given injected energy, the ratio of the expansion time of the bubble to its implosion time drops with increasing in liquid viscosity. The bubble dynamics are then governed by liquid viscosity. Moreover, the transition of the bubble dynamics from the inertial regime to the viscous one has been experimentally observed (as far as the present authors are aware) for the first time. This transition can be explained by a refined analysis of the Rayleigh–Plesset model of bubble dynamics. The bubble dynamics regime can be deduced from a Reynolds number (ReTp) versus elasticity number (Σ) diagram, where four zones can be distinguished. Each zone corresponds to a particular regime: inertial regime with only one growth and collapse stage, inertial regime with at least one bubble rebound, viscous regime, and finally, a regime where a jet of hot liquid is produced. All experimental results are well distributed into the good part of this diagram.