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Objectives. The aim of this study is to investigate depth dependent changes of polymerization process and kinetics of visible light-curing (VLC) dental composites in real-time. The measured quantity -"ion viscosity" determined by dielectric analysis (DEA) -provides the depth dependent reaction rate which is correlated to the light intensity available in the corresponding depths derived from light transmission measurements. Methods. The ion viscosity curves of two composites (VOCO Arabesk Top and Grandio) were determined during irradiation of 40 s with a light-curing unit (LCU) in specimen depths of 0.5/0.75/1.0/1.25/1.5/1.75 and 2.0 mm using a dielectric cure analyzer (NETZSCH DEA 231 with Mini IDEX sensors). The thickness dependent light transmission was measured by irradiation composite specimens of various thicknesses on top of a radiometer setup. Results. The shape of the ion viscosity curves depends strongly on the specimen thickness above the sensor. All curves exhibit a range of linear time dependency of the ion viscosity after a certain initiation time. The determined initiation times, the slopes of the linear part of the curves, and the ion viscosities at the end of the irradiation differ significantly with depth within the specimen. The slopes of the ion viscosity curves as well as the light intensity values decrease with depth and fit to the Lambert-Beer law. The corresponding attenuation coefficients are determined for Arabesk Top OA2 to 1.39 mm -1 and 1.48 mm -1 , respectively, and for Grandio OA2 with 1.17 and 1.39 mm -1 , respectively. For thicknesses exceeding 1.5 mm a change in polymerization behavior is observed as the ion viscosity increases subsequent to the linear range indicating some kind of reaction acceleration. Significance. The two VLC composites and different specimen thicknesses discriminate significantly in their ion viscosity evolution allowing for a precise characterization of the curing process even with respect to the polymerization mechanism.