Porosity in the pastes of composite resins before and after polymerization has been measured. The influence of the diameter of the extrusion orifices and the spatulation of the chemically-activated pastes has been evaluated. Considering that thin layers of these materials are transparent, the investigation has been carried out mainly by analysis of approximately 300 microns or lower thickness layers, that are still transparent under the microscope. Generally, there was a significant dependence of the porosity on the orifice diameter. However, this rule showed some exceptions. As far as the chemically-activated pastes were concerned, in all examined cases the number of small size (10-40 microns) porosities increased after spatulation.