In order to prepare cured thin films thicknesses in the range of 90-300 nm, epoxy-amine mixtures of different concentrations in toluene are spin-cast onto oxidized silicon substrates. The glass transition temperature of the cured thin films is measured by microthermal analysis, revealing the existence of two distinct glass transitions temperatures for all the samples due to amine segregation at air/film interface. These transitions are ascribed to two layers. The upper layer properties of the film due to the air/polymer interface are independent of the film thickness. This layer is around 30 nm thick, and its glass transition temperature is about 97 degrees C and matches to a constant amine/epoxy composition at the air/film interphase. Consecutively, the film thickness reduction induces an increase of the epoxy excess in the sublayer, promoting side epoxy reactions revealed by aliphatic ether bonds formation. Thus, significant increase of the sublayer T-g, in comparison to the T-g of the bulk sample with an equivalent amine/epoxy ratio at the same amine conversion rate, is mainly due to these new ethers bonds for film thickness less than 160 nm. These bonds, surface catalyzed, are created at low temperature by the consumption of the epoxy excess. Etherification enhancement is finely controlled by the amine-epoxy off stoichiometry in the sublayer tuned by its thickness and mainly results in glass transition temperature increase at lightly amine conversion. At complete epoxy and high amine conversion reached by postcuring, etherification in the sublayer leads to a constant amount whatever the thickness of the sample. The glass transition temperature of the sublayer postcured is around 180 degrees C, equivalent to the bulk sample and does not bring out confinement effects.