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Glass ionomer cements (GICs) are currently the first-choice materials in minimally invasive dentistry and are widely used in paediatric dentistry. The objective of this study was to evaluate the influence of viscosity and mixing on porosity and the relationship between the porosity and microhardness of GICs. Nine GICs were used: EQUIA® Forte Fil, Ionostar®Plus, 3MTMKetacTM Universal AplicapTM, Riva Light Cure HV®, Riva Silver®, Riva Protect®, VitremerTM, VitrebondTM Plus and Activa BioActive RestorativeTM. Samples were prepared from each material and images were obtained by scanning electron microscopy (SEM). Using software, the number and diameter of pores, area, and percentage of area they occupied were measured. After SEM analysis, the samples were used to make surface microhardness (SMH) measurements. Ionostar® Plus, EQUIA® Forte Fil and Riva Light Cure HV® had the highest number of pores. Riva Protect® had the largest pores and the highest percentage of pore-occupied area, while Activa BioActive RestorativeTM and VitrebondTM Plus had a smaller pore size and a smaller percentage by area. Riva Silver® and EQUIA® Forte Fil showed the highest microhardness values and VitrebondTM Plus and VitremerTM the lowest. In conclusion, when the cement viscosity is low, the type of mixture determines the degree of porosity, but has less influence when viscosity is high. There was no correlation between microhardness and the porosity of GICs.