The incorporation of thixotropic agents to prevent slumping during the silicone packing procedure may alter some favorable mechanical properties of the silicone elastomers and could possibly influence the success of the prosthesis.The purpose of this in vitro study was to evaluate the effect of adding thixotropic agents on the mechanical properties of 3 commonly used silicone elastomers.Specimens of 3 maxillofacial silicones (M511, Z004; Technovent Ltd, and A2000; Factor II Inc) were prepared according to the manufacturers' instructions. Tear and tensile strength values and percentages of elongation and hardness were evaluated for each material with and without thixotropic agents. Data were analyzed using 1-way ANOVA and the Bonferroni post hoc test (α=.05).Results showed that the 3 types of silicone elastomers had significantly different (P<.001) tensile and tear strength and hardness values and percentages of elongation. Z004 silicone showed the highest tensile and tear strength followed by A2000 and M511, regardless of the addition of thixotropic agent. The addition of a thixotropic agent decreased the tear strength (P<.001) but not the tensile strength for all types of silicone. Percentage of elongation was the highest in M511 and the lowest in A2000 and was significantly higher (P<.001) for silicones with no added thixotropic agent. Hardness was highest in A2000 and lowest in M511. Adding a thixotropic agent decreased hardness significantly (P<.001) for Z004 and A2000 only.The incorporation of thixotropic agents into the 3 maxillofacial silicone elastomers used in this study reduced some favorable mechanical properties, particularly tear strength and percentage of elongation. Z004 showed superior mechanical properties among the 3 tested silicones.