Background: enamel demineralization is a common side effect of orthodontic therapy with fixed braces. The aim of the present in vitro study was to compare a conventional adhesive system and a modern adhesive system (APC Flash-Free [FF] technology) with regard to the demineralization of enamel byStreptococcus sobrinus(S. sobrinus). Methods: this in vitro study included premolar teeth and compared APC FF adhesive brackets (Group A,n= 15) with conventional adhesive brackets (Group B,n= 15) from the same company. Specimens were incubated with a positive control group (PCG,n= 5) and a negative control group (NCG,n= 5) in anS. sobrinussuspension for three weeks. To evaluate the grade of enamel demineralization, the samples were analyzed using a polarizing microscope. Results: the test specimens of group B with conventionally bonded bracket adhesive showed significantly greater (+10.8 μm) demineralization with regard to the penetration depth of the demineralization than the PCG (p= 0.012). Thus, there was a difference from group A with the new bracket adhesive of the FF brackets (+7.29 μm). Significantly, demineralization was more pronounced cervically than coronally in both groups, and it occurred cervically more frequently than grade 3 demineralization (p= 0.001). Conclusions: it seems plausible that new orthodontic bracket adhesives and the modern FF adhesive system positively contribute to the reduction in enamel demineralization during orthodontic treatment.