Aim: To evaluate the antimicrobial effect of ozone as a disinfectant for alginate and polyether rubber base impression materials and to study the effect of ozone gas and ozonated water on the dimensional changes and surface topography of the aforementioned hydrophilic impression materials.Materials and methods: Two hydrophilic impression materials (alginate and polyether rubber base) were used to prepare dental impression samples to study the disinfecting effect of ozone gas and ozonated water on the selected microorganisms (Candida albicans, Escherichia coli and staphylococcus aureus) for time intervals of (2,3 and 4 minutes) respectively. Five impression specimens were prepared for each impression material, poured in dental stone and after setting, evaluation of the dimensional changes and surface topography were performedto obtain the control groups measurements before ozone application. The study groups were: Group A (alginate disinfected with ozone gas), group B (polyether disinfected with ozone gas), group C (alginate disinfected by mixing with ozonated water), group D (polyether disinfected by immersion in ozonated water) and group E (alginate disinfected by immersion in ozonated water). Five impression specimens for each of the three time intervals (2, 3 and 4 minutes) were prepared for each group. After being subjected to ozone treatments, stone casts were obtained. The effect of different ozone treatments on the dimensional changes and the surface roughness (Roughness average Ra) of the impression materials specimens was studied by evaluating the corresponding dental stone casts using travelling microscope and profilometer respectively.Results: The use of ozone as disinfectant has successfully eliminated the three microorganisms used. There was a direct relation between increasing time and the efficacy of disinfection whatever the ozone form used. Regarding the dimensional changes, ozone treatment for 4 minutes revealed that alginate impression material disinfected by immersion in ozonated water showed the statistically significantly highest mean value, followed by alginate disinfected with ozone gas then alginate mixed with ozonated water. However, polyether impression material immersed in ozonated water showed lower mean dimensional changes value. Polyether disinfected with ozone gas showed the statistically significantly least mean dimensional changes value. Regarding the surface topography, ozone treatment caused an increase in the mean roughness average (Ra) values with the increase in time. The alginate mixed with ozonated water showed the highest mean Ra along the different times followed by alginate immersed in ozonated water and the least mean Ra was obtained with alginate disinfected with ozone gas. Regarding polyether rubber impression disinfection by ozone gas and by immersion in ozonated water showed no statistically significant difference in Ra within 2 and 3 minutes while at 4 minutes, the Ra of polyether rubber base impression disinfected with gas was lower.Conclusions: Ozone is a powerful safe disinfectant for hydrophilic dental impressions. There was a direct relation between increasing time of disinfection and the efficacy of disinfection with microorganisms used whatever the ozone form used. Immersion of alginate and polyether impressions in ozonated water had the highest disinfection effect but it produced the highest dimensional changes. The least surface roughness was obtained when subjecting alginate and polyether impressions to ozone gas. The best results could be obtained using ozone gas disinfection for 4 minutes for both impressions materials.