Purpose: To evaluate the phase separation, extent of dentin demineralization, and curing efficiency of one-step self-etching adhesives placed on dentin and on inert substrate. Materials and Methods: The adhesives tested were G-Bond (GB), iBond (IB) and Clearfil S3 Bond (SB). Two groups (2 x 3) of dentin and inert (gold) specimens were prepared for each product. The films formed on inert surfaces were subjected to mild (1st group) or strong air drying (2nd group) and examined with near- and mid-FTIR microspectroscopy. On dentin (1st group), the extent of demineralization (%DM) was evaluated before and after adhesive treatment plus rinsing with water and acetone by mid-FTIR microspectroscopy. The films formed after strong air drying (2nd group) and 20 s of light curing were rinsed with acetone, and the degree of cure (%DC) was measured by micro-multiple internal reflection FTIR spectroscopy. ANOVA was used to assess the statistically significant differences in %DM and %DC (α = 0.05). Results: In GB and IB, mild air drying produced more water droplets and monomer separation than strong air drying. SB demonstrated the least separation. IB induced the highest %DM. All the adhesives demonstrated the greatest %DC on dentin. On both substrates SB showed the highest %DC values. Conclusion: Phase separation including water and/or monomers was found in all the adhesives. The extent of dentin demineralization and degree of cure varied among the products tested. The degree of cure was higher on dentin than on the inert substrate. Keywords: self-etching adhesives, phase separation, FTIR spectroscopy and imaging, demineralization, degree of cure