Background and Aim: Disinfection of dental impression materials can alter their dimensional stability, which in turn can affect the accuracy of dental casts. This study aimed to determine the effect of three disinfectants on the dimensional changes of irreversible hydrocolloid impression materials and addition silicones. Materials and Methods: In this in vitro experimental study, forty alginate and 40 addition silicone impressions were made using special trays. The impressions were disinfected by the spraying method for 10 minutes except for the control group. Then, they were poured with type IV dental stone. The prepared casts were maintained for 24 hours in a similar condition. Next, the dimensions and the external distance between two dies were measured at three time intervals using a digital caliper with 0.01 mm accuracy, and the average values were recorded. The yielded measurements were compared with each other and with metal models. One-way ANOVA and one sample t-test (α=0.05) were used for data analysis. Results: No significant difference was observed between the groups (P>0.05). Comparison of all average dimensions of the casts with the metal model showed that none of the groups had a significant difference except for the alginate group that showed an increase in the height of the larger die. There was a significant difference in the rest of the dimensions (P<0.05). Conclusion: Disinfection of alginate and addition silicone impression materials with 0.5% sodium hypochlorite, Epimax, and Deconex did not have any significant effect on the cast dimensions.