Taking into account the temperature rise under the electron beam, phase separation in a supersaturated B2O3‐PbO glass is shown to be enhanced by electron irradiation. Coagulation of PbO‐rich particles is observed duringinsituelectron microscopic experiments. This mechanism of coalescence is discussed. Using the Smoluchowski model of coagulation we show that the glass viscosity is drastically lowered: typically eight to nine orders of magnitude at 270 °C and only one order at 420 °C. This effect is shown to disappear almost instantaneously when irradiation is stopped.