The posteffects in the polymerization of methyl methacrylate at low monomer conversions are studied. The experimental two-step method consists of a short initial photoinduced polymerization period and subsequent register of the monomer concentration evolution in the dark. After switching off the light, the radical concentration drops sharply, but a nonnegligible concentration of persistent radicals continues the polymerization reaction for long periods of time, yielding ultrahigh molecular weight materials (over 10 000 monomer units). By a fitting procedure applied to the conversion-time curves obtained during the dark period, average values of the termination rate coefficients, < k(t)>, scarcely exceeding 10 000 L mol(-1) s(-1), and radical concentrations at the beginning of the postpolymerization about 10(-8) mol L-1 are determined. The reliability of these fitting results is checked by the design of reactions with similar polymerization rate but under steady-state conditions. The molecular weight of the polymers synthesized during the dark period is controlled by the transfer reactions to the monomer and to the initiator. Both transfer reaction constants are estimated by using a method based on the analysis of the instantaneous chain length distribution. The extremely low < k(t)> values obtained by the analysis suggest that the power-law exponents which describe the chain-length dependency of k(t) should be higher than the unity for long radicals even in dilute solution (i.e., at low monomer conversions).