Two different types of colloidal gold nanoparticles, produced as hydrosol, are recognized from their electronic absorption spectra. Gold nanorods with aspect ratio in the range between 2 and 4 and average diameter between 10 and 15 nm are produced using AuCl3 and hydroxylamine. The surface plasmon resonance (SPR) band of gold nanorod undergoes a blue shift of about 50 nm on treatment with ozone. On the other hand, spherical gold nanoparticles with average diameter around 20 nm are produced from AuCl3 reduced with black tea infusion. When treated with ozone the SPR band of the spherical gold nanoparticles show a red shift of more than 50 nm. This demonstrates that the shape of the gold nanoparticles deeply affects their response in the interaction with chemicals in view of the potential analytical application of gold nanoparticles as active materials in chemical sensors. A theoretical rationalization of the results is proposed