Thiol-passivated gold nanoparticles (AuNPs) of a relatively small size, either decorated with. chromophoric groups, such as a phthalimide (Au@)PH) and benzophenone (Au@BP), or capped with octadecanethiol (Au@ODCN) have been synthesized and characterized by NMR and UV-vis spectroscopy as well as transmission electron microspcopy (TEM). These NPs we re irradiated in chloroform at different UV-wavelengths using either a nanosecond laser (266 and 355 rim, ca. 12 mJ/pulse, 10 ns pulse) or conventional lamps (300 nm < lambda < 400 nm and ca. 240 nm < lambda < 280 nm) and the new AuNPs were characterized by X-ray and UV-vis spectroscopy, as well as by TEM. Laser irradiation at 355 rim led to NP aggregation and precipitation, while the NPs were photostable under UV-A lamp illumination. Remarkably, laser excitation at 266 nm induced a fast (minutes time-scale) increase in the size of the NPs, producing huge spherical nanocrystals, while lamp-irradiation at UV-C wavelengths brought about nano networks of partially fused NPs with a larger diameter than the native NPs.