Objectives: The aim of this study was to compare the in-vitro caries inhibition of two polyacid-modified composite resins (PMCRs).Methods: Three standardized class V cavities were prepared in eight extracted human molar teeth. Two of these cavities in each tooth were restored with Compoglass and Dyract according to manufacturers' guidelines. A conventional glass-ionomer cement restoration (Chemfil II) was placed in the third cavity in each tooth as a control. The teeth were submerged in an acid gel (lactic acid, gelatin, thymol, pH 4.5) for 28 days and then resin embedded. Ground sections were examined at 160 magnification for enamel surface lesion depth, dentine surface lesion depth, wall lesions and evidence of caries inhibition.Results: There was no significant difference between the depths of enamel or dentine surface lesions between the two compomer and conventional glass-ionomer materials. However, both PMCRs exhibited greater wall lesions than the conventional glass-ionomer material indicating less caries inhibition.Conclusions: These results suggest that PMCRs provide less caries inhibition than glass-ionomer cements.