The thicknesses and densities of coatings prepared by atomic layer deposition (ALD) and by other deposition techniques are important to understand their optical, electrical and mechanical properties. Specular X-ray reflectivity (XRR) is a powerful tool that is capable of nondestructively extracting layer thicknesses with angstrom resolution, and what is distinct, determining the layer densities through modeling structures with simulation programs. Unfortunately, problem with modeling usually arises when other layers form at the surface or interfaces, and it is also common to other nondestructive techniques such as ellipsometry. A recent development1 has shown that with differentiating the specular XRR intensity with respect to vertical reciprocal space coordinate, a Fourier transform (FT) approach can provide straightforward information about the layer thicknesses in multilayer systems, which will make it easier to build the XRR simulation model. Therefore, a novel combination of FT and specular XRR simulation would be quite efficient in layer thickness extraction compared with other conventional techniques (e.g. transmission electron microscope (TEM) and ellipsometry). In this study, the novel XRR approach, together with other techniques including optical microscope (OR), spectroscopic ellipsometry (SE), TEM, energy dispersive X-ray spectroscopy and glancing incidence X-ray diffraction, was utilized to characterize the effect of annealing on optical properties of Al2O3/TiO2/Al2O3 multilayer antireflection stack. The films were deposited on (0 0 1) Si wafers using ALD which is a good candidate for multilayer antireflection coatings, due to the self-limiting nature of chemical reactions resulting in the precise control of film thickness and large-area uniformity. The thickness values of the as-deposited layers determined by XRR agree quite well with those obtained from OR, SE and TEM, with the difference below uncertainty levels. This, together with the good correlation between layer densities extracted from XRR and refractive indices measured by OR and SE, demonstrates the validity of the XRR approach. On the other hand, it is revealed that air annealing at 400 °Ϲ for 10 min induces the densification of the TiO2 layer as well as the initiation of crystallization. Meanwhile, both Al2O3 layers keep amorphous after annealing, with the thickness of top Al2O3 layer decreased that is likely due to interdiffusion of Al and Ti atoms between the top two layers, while the thickness of bottom Al2O3 layer increased accompanied with a decrease in density, which is probably attributed to the diffusion of Si atoms from the substrate into layers. Reference 1B. Poust, R. Sandhu, and M. Goorsky, Phys. Status Solidi A. 206, 1780 (2009).