In this article, the correlation between PA-FTIR and DSC techniques was carried out to obtain a more comprehensive interpretation for the fiber-matrix interface of glass fiber-reinforced PET and the dependence on annealing temperature. In contrast to the thermal behavior of an unannealed matrix or an annealed one at low temperatures (below 110 degrees C), glass transition is barely perceptible and no crystallization peak can be found in the composite. Therefore, the lack of these thermal characteristics indicates the absence of the primary isomerization in the matrix. Similarly to the isolated matrix, low-melting and high-melting peaks appear in the thermograms of the annealed composite beyond 135 degrees C, but the size and perfection of crystallites seems to be less in the composite. Conversely to the spectroscopic results, the crystallinity values of the composite tend only to coincide with those of the annealed isolated matrix at temperatures beyond 150 degrees C, and below this temperature, they are always lower, until the primary isomerization of the matrix. A comparison between the results obtained with both techniques seems to indicate an improvement on the fiber-matrix interface in which an interfacial structure with low degree of perfection changes to more perfect crystals due to the annealing treatment.