Reactions between thin films of CA(2) and (0001)-oriented alpha-Al2O3 have been studied using a combination of microscopy techniques. Thin films of amorphous CA(2) were deposited on sapphire substrates by pulsed-laser deposition at 900 A degrees C in an oxygen ambient atmosphere. After deposition, the reaction couples were heat treated in air for various times either at 1300 or 1400 A degrees C. Atomic-force microscopy was used to monitor changes in the microstructure of the films. Interfaces between the different regions were examined by transmission electron microscopy (TEM) of cross-sectional samples prepared by focused ion-beam milling. The CA(2) films had dewetted the substrate surface as a result of the heat treatment. An interfacial reaction layer was observed between the dewetted CA(2) droplets and the substrate. The structure of this reaction layer was found to be consistent with gamma-Al2O3 by computer analysis of high-resolution TEM images. There is a perfect epitaxy between the interfacial layer and the substrate. For the samples heat treated for longer times, hexagonal features were found on the substrate surface. The presence of these features on (0001)-oriented alpha-Al2O3 suggests that CA(6) platelets form by the transformation of the interfacial reaction layer. The results are discussed in relation to the crystallization behavior of the various calcium aluminate phases and the equilibrium-phase diagram of the CaO-Al2O3 system.