This study was performed to evaluate adaptation of sonic-fill versus bulk-fill resin composite restorations. Materials and Methods: A total number of 80 standardized Class II cavities were prepared on both proximal walls of total number of 40 premolars.  The teeth were randomly divided into two equal groups, of 20 teeth each, according to the material; either SonicFill or X-tra Fil resin composite in bulk (5mm). Each group was divided into two equal subgroups, of 10 teeth each, according to sonic vibration applied; either application of resin composite using sonic vibration or without sonic vibration. Each subgroup was divided into 2 classes, of five teeth each (with 10 proximal restorations), according to the aging duration; either after 24 hours or 12 months in artificial saliva at 37˚C. The specimens were observed after tracing by caries detecting dye under 70X magnification. Results: There was no statistically significant difference between Sonicfill and X-tra Fil resin composite materials either used with the use of sonic vibration or manual packing. Moreover, aging has no significant effect on voids formation in the composite/tooth interface. Conclusion: Application techniques either using sonic application or manual packing. Moreover, aging revealed no difference for both materials regarding adaptation.