This study aimed to evaluate whether preheated resin-based flowable restoratives would show increased hardness and softening susceptibility after an early cariogenic challenge. Fluroshield- Yellowed, Bioseal, Wave, Master Flow, Fluroshield-White, Conseal F, Filtek Z350 Flow, and Opallis Flow were tested. Preheating was performed using a microwave device. Five specimens of each preheated or room temperature material (n = 5) were fabricated. Hardness was assessed before and after a cariogenic challenge. The analysis was done by two-way analysis of variance (ANOVA) with repeated measures (cariogenic challenge) and Tukey's test for multiple comparisons (