The ultrafast dynamics of electronic bubble formation upon excitation of the A(3ssigma) Rydberg state of NO trapped in solid H-2 and D-2 has been studied by femtosecond pump-probe spectroscopy. The evolution of the spherical bubble is followed in real time by means of a probe pulse, which maps the transient configurations via transitions to higher-lying Rydberg states. It is found that bubble formation is a one-way process and no oscillations of the bubble are observed. In addition, thermalization of the system occurs on the time scale of bubble formation. In the process, there is a net energy flow away from the excited center and 0.55-0.6 eV leave the first shell around the impurity. We directly extract from the experimental data the time dependence of the bubble radius, which we represent by a rising exponential with time constants of 300+/-50 fs in solid H-2 and 410+/-30 fs in solid D-2 to reach a final radius of similar to5 Angstrom. This is confirmed by simulations of the transients. The different energy dissipation mechanisms in the expansion of the bubble are discussed and we suggest that emission of a sound wave is the dominant one.