The surface chemical reactions occurring during the laser-assisted deposition of gold from dimethyl gold hexafluoroacetylacetonate were studied using a photothermal modulation technique. A modulated argon ion laser was used to heat an area roughly 1 mm in diameter on an alumina substrate. Products of the photothermal reaction along with reactants were sampled through a 150×150 μm orifice located at the center of the deposit and were then introduced directly into a quadrupole mass filter. An inert buffer gas was required to confine changes in the reactant and product concentrations to the region surrounding the sampling orifice. Since the reactant concentration at the surface decreased when the laser was turned on while the product concentration at the surface increased when the laser was turned on, modulating the surface reaction rate by modulating the argon ion laser beam and employing phase-sensitive detection allowed the identification of reaction products. The behavior of the system was described qualitatively using a theory for coupled reactant transport to the deposit surface and surface chemical reaction.