The aim of this in vitro study was to evaluate the influence of infiltrant composition and application frequency on micro-hardness and lesion progression after resin infiltration of artificial enamel lesions.In each of 100 bovine enamel samples, three artificial caries lesions were created (pH=4.95, 50 days). After etching two of the lesions (37% phosphoric acid) specimens were randomly allocated to five infiltrants (four experimental infiltrants with different monomer and solvent compositions and penetration coefficients, and one commercial infiltrant [Icon, DMG]). Lesions were then infiltrated and light-cured, and infiltration repeated afterwards for one of the lesions. Infiltrated samples were cut into halves, with one half being demineralised for further 50 days. Micro-hardness (VHN) and integrated mineral loss (ΔZ) were evaluated at baseline and after second demineralisation. Repeated measures ANOVA and paired t-tests were used to analyse influence of material composition and application frequency on micro-hardness and lesion progression (integrated mineral loss difference ΔΔZ).Resin infiltration significantly increased micro-hardness and reduced lesion progression compared to untreated artificial lesions (p0.05, ANOVA). In contrast, twice application resulted in significantly increased micro-hardness and demineralisation resistance of infiltrated lesions (p<0.05, ANOVA).Resin infiltration significantly improves micro-hardness and demineralisation resistance of enamel lesions; these effects are significantly enhanced if resins are applied twice. Experimental resins did not outperform the commercial infiltrant.This in vitro study demonstrated that resin infiltration significantly increases both micro-hardness and demineralisation resistance of enamel caries lesions. Twice application of the infiltrant seems to increase these effects. In contrast, the composition of the infiltrant had no significant influence on investigated parameters.