Purpose: To evaluate the bond strengths of three polyacid-modified resin composites and one resin-modified glass-ionomer cement to caries-simulated primary dentin after 18-months' water immersion. Methods: Dyract AP, Compoglass F, F2000 and Vitremer were used as test materials. Standardized preparations cavities with a C-factor of 5 were prepared in primary molars. Half of the cavities were stored in a demineralizing media for 7 days to simulate caries-affected dentin. Thereafter, all preparations were restored with the test materials, after which the specimens were stored in water for 18 months, changing the water every week. The specimens were subjected to a microtensile test and all bond strength data (in MPa) were statistically analyzed using one-way analysis of variance (ANOVA) and Tukey HSD tests at P < 0.01. Results: The mean microtensile bond strength to normal primary dentin specimens were 11.2 +/- 1.5 (Dyract AP), 10.6 +/- 2.1 (Compoglass F), 10.8 +/- 1.1 (F2000) and 7.3 +/- 1.6 (Vitremer), while those to the caries-simulated dentin were 4.1 +/- 0.7 (Dyract AP), 3.0 +/- 0.6 (Compoglass F), 2.3 +/- 0.9 (F2000) and 1.9 +/- 0.5 (Vitremer). SEM evaluation of the unfractured specimens revealed evidence of gap formation in the adhesive interface in all samples.