We have investigated the thermal and photon-induced chemistry of several aromatics adsorbed on smooth and roughened Ag(111) surfaces at low temperatures (110 K). Pyrazine, pyridine, 3-chloropyridine, and chlorobenzene quantitatively desorb from Ag(111) near 200 K. After roughening the surface with 2 kV Ar ion bombardment, the breadth of the molecular desorption curves increases, and there is a high-temperature tail due to desorption from defect sites produced by the roughening process. Upon UV photolysis of 1 monolayer, chlorobenzene and 3-chloropyridine photodissociate on smooth and rough Ag surfaces, whereas no detectable photoreactions were observed for pyrazine or pyridine adsorbed on either surface. For the molecules that did undergo photodissociation, a shift to lower energy in the photodissociation threshold was observed on the rough surface relative to the smooth : 3.3 eV versus 3.5 eV for chlorobenzene and 3.5 eV versus 3.9 eV for 3-chloropyridine. We postulate that the decrease in the photodissociation threshold is due to defect sites produced by surface roughening, through either excitation of the surface plasmon resonance, which is allowed on the rough but not the smooth surface, or a more localized excitation near defect sites. Excitation in the molecular absorption band shows that the photodissociation yield is enhanced for 3-chloropyridine and quenched for chlorobenzene on the rough surface relative to the smooth. Theoretical calculations suggest that there is a decrease in the decay rate for 3-chloropyridine and an increase in the decay rate for chlorobenzene on the rough surface compared to the smooth. Differences between the quenching rates for 3-chloropyridine and chlorobenzene may be related to the different molecular orientations of these two molecules on the surface.