Methyl methacrylate is polymerized under near-isothermal conditions in a 1-I stainless steel batch reactor having a glass bottom and stirred with an anchor agitator. A camcorder is used to obtain images of the entrapped vapor bubbles at different times so as to identify the point, Bi, where stable clusters of vapor bubbles first (incipient) get formed. This is being referred to as the transition point. Data on the monomer conversion, x(m,Bi), the weight average molecular weight, M-w,(BI), and the viscosity, eta(Bi), of the reaction mass at this point have been generated for a variety of experimental conditions. It is observed that the pressure above the liquid reaction mixture and the RPM of the stirrer have little effect on the results in the ranges studied. Correlations have been developed for x(m,Bi), M-W,M-Bi, and eta(Bi), relating these to the initiator loading, I-o, and the constant set-point temperature, T. Results are compared with those obtained earlier for reactors stirred with ribbon agitators. The transition point is useful for the design of the first stage of polymerization reactors. POLYM. ENG. SCI., 49:2309-2314, 2009. (C) 2009 Society of Plastics Engineers