The thermal aging of filled and unfilled vinyl ester was performed at 180°C under air and was followed at the molecular (Fourier Transform InfraRed Spectroscopy) and macromolecular (Dynamical Mechanical Analysis) scales. Aluminum trihydrate filler was observed to be stable in the investigated conditions, which simplifies the analysis of aging. At the molecular scale, oxidation is visible by the broadening of the carbonyl band, the depletion of hydroxypropyl ether, and the loss of mass. Subglassy relaxation and glass transition (Tg) of unaged samples were also characterized by dynamic mechanical analysis. After aging, a clear crosslinking mechanism was evidenced by an increase in Tg. The possible oxidation mechanisms are discussed.