Frequency dependent dielectric measurements (FDEMS), dynamic mechanical rheometer measurements (DMA) and modulated differential scanning calorimetry (DSC) are applied to observe the formation of a spatial heterogeneity in dimethacrylate/divinylbenzene (D121/DVB) and dimethylacrylate/styrene (D121/St) network systems. The structure evolution information during the reaction, such as T, the dielectric relaxation and the dynamic mechanic relaxation obtained from these measurements for both systems are compared. The result shows that a spatial heterogeneity does exist in both systems. This is based on the existence of two distinct separate relaxation peaks in the dielectric spectrum, two T-g's in the DSC profile and the dynamic mechanical analysis. These two regions are ascribed to the formation of microgel regions as previously identified by dynamic light scattering and monomer/oligomer regions. The experimental spectra are affected by the comonomer structure. The breadth of the relaxation distribution of the spatial heterogeneity of the microgel region in the DVB comonomer system is extremely large compared to the large breadth in the styrene system. (c) 2005 Elsevier Ltd. All rights reserved.