We present a study of dispersion polymerization of methyl methacrylate in supercritical CO2 (SC-CO2) at 330 bar in situ by turbidimetry. All experiments have been done in the presence of the macromonomer poly(dimethylsiloxane)-monomethacrylate (PDMS-MA) which acts as a stabilizer. The formation of particles of poly(methyl methacrylate) (PMMA) can be monitored quantitatively by turbidimetry because the degree of swelling by SC-CO2 as well as the refractive index of these particles is known accurately. The turbidity spectra were measured in the range 400-950 nm. The number density N/V and the diameter o(r) could be obtained as a function of time in the earliest stage of the dispersion polymerization with a time resolution of ca. 0.1 s. Moreover, the mass of polymer m(p)(t) could be deduced by means of which a full kinetic analysis could be performed. Special attention has been paid to the size distribution of particles that is shown to play an essential role in the treatment of turbidimetric data. It is demonstrated that the locus of polymerization in the early stage studied here is the homogeneous phase. N/V(t) raises quickly in the nucleation period (stage 1) and remains then constant (stage II). The diameter of the critical nuclei, i.e., sigma(r), measured in stage I is ca. 150-170 nm. All data obtained are in semiquantitative agreement with the model proposed by Paine.