Purpose: The present study was undertaken to determine the content and the constituent elements of inorganic fillers as well as the radiopacity of 15 recently introduced bonding agents and to compare their radiodensities with those of enamel and dentin by using a transmission photodensitometer. Materials and Methods: The content of inorganic fillers was determined by the ashing method. In addition, x-ray microanalysis was used to identify the constituent elements of the inorganic fillers. In the measurement of the radiopacity, specimens 10 mm in diameter and 0.9 mm in thickness were fabricated from the 15 different bonding agents (n = 3 each) for a total of 45 specimens. Human premolars were longitudinally sectioned into 1.0-mm-thick slices to include both enamel and dentin, then ground to final thickness of 0.9 mm. The optical densities of enamel, dentin, the bonding agents, and an aluminum (Al) step wedge were obtained from radiographic images by transmission photodensitometry. The Al equivalent (mm) for each material was calculated by using the linear regression equation of the log of the normalized optical density and Al thickness (mm) obtained from the step wedge. A linear regression of the logarithm of the normalized optical density and Al thickness (mm) was plotted. Results: The inorganic filler content ranged from 0.0 wt% to 43.5 wt%. Silicon and aluminum were the main constituent elements of the inorganic fillers; however, the resins contained different kinds of inorganic fillers. These elements have high and low atomic numbers. The surface reaction type pre-reacted glass-ionomer (S-PRG)-filled bonding agent (FII) had a filler content of 43.5 wt% and contained B, Si, Sr, Na, F and Al as its elemental constituents. The relative radiopacities expressed as equivalent thickness of Al were ranked ordinally. Fourteen of the 15 adhesive resins had radiopacity values that fell below that value for dentin, whereas the FII resin showed a radiopacity that was equal to or greater than that of dentin but below that of enamel (p 0.05). Conclusion: The radiopacity of bonding agents depended on their filler content and the presence of heavier atoms of elements in the filler. Palabras clave: bonding agent, filler, radiopacity, densitometer, Al equivalent