The heterogeneously catalyzed aerobic selective oxidation of naturally occurring lignan hydroxymatairesinol (HMR) to another lignan oxomatairesinol (oxoMAT) was carried out at 70 degrees C and atmospheric pressure over gold nanoparticles with different sizes supported on alumina prepared from organometallic precursor by the sal-gel method. Precursor of gold nanoparticles (i.e. gold hydroxide) was supported on alumina by deposition-precipitation with urea from HAuCl4 aqueous solution followed by washing with ammonium hydroxide. The size of the supported gold nanoparticles was changed by either treatment under hydrogen flow at different temperatures or exposure to ultraviolet (UV) irradiation at the wavelength of 254 nm. The catalysts were characterized by inductively coupled plasma atomic emission spectroscopy (ICP-AES), nitrogen adsorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectra (XPS), and UV-visible spectroscopy (UV-vis). Structure sensitivity in HMR oxidation was studied showing that the reaction rate passes through a maximum when the gold cluster size is ca. 4 nm.