The polymerization of methyl methacrylate (MMA) is carried out under near-isothermal conditions in a 1-liter stainless steel stirred batch reactor with a glass bottom. A ribbon agitator is used to study the incipient stable-bubble formation point, Bi, during the bulk polymerization of MMA. A camcorder has been used to get images of the bubbles from the beginning of the polymerization till just beyond the incipient stable-bubble formation point. Experimental data on the monomer conversion, X-m,X-Bi, the weight average molecular weight, M-w,M-Bi, and the viscosity, eta(Bi) of the reaction mass have been generated at this important point. Data have been taken at several values of the (near-isothermal) temperature, initiator loading, reactor pressure, and the RPM of the ribbon stirrer. No significant effect of pressure and the RPM of the stirrer were observed over the ranges of these variables studied. Correlations have been developed for x(m,Bi), M-w,M-Bi, and eta(Bi), relating these to the initiator loading and the temperature. POLYM. ENG. SCI., 49:930-936, 2009. (C) 2009 Society of Plastics Engineers