Little is known about the interfacial photochemistry of transition metal cations and chromophores relevant to atmospheric aerosols. We report herein water uptake and in situ surface-sensitive spectroscopic studies on the photosensitized transformation of solid gallic acid (GA), externally mixed with FeCl₃ as a photosensitizer, under dry and humid conditions of <1% and 30% relative humidity (RH), respectively. GA is a hydrolysis product of tannic acid, a model macromolecule for humic-like substances (HULIS) in aerosols and aged polyaromatic hydrocarbons (PAH). Water uptake on GA and GA/FeCl₃ mixture films was quantified using a quartz crystal microbalance (QCM) as a function of %RH (<1–60%). Results indicate continuous multilayer formation of adsorbed water with no phase transitions, with a monolayer of adsorbed water forming around 30 and 12%RH, respectively. Photochemical studies were conducted using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Spectra were collected as a function of irradiation time (4 h), mass fraction of FeCl₃ (0.5–3%) using irradiance of simulated solar light equivalent to 120 Wm⁻² at 555 nm. Difference absorbance spectra show changes to GA functional groups suggesting the formation of organochlorine compounds in the condensed phase with their signature v(CC) at 1601 cm⁻¹, and release of CO₂. Potential halogenation pathways of GA in the presence of Fe³⁺ are discussed based on well-known aqueous phase chemistry. These pathways along with our results also suggest the release of volatile organochlorine compounds and Cl₂ gas. Apparent first order rate constants, k_s, of the photosensitized reactions were derived from kinetic curves of the most intense positive and negative spectral features at 1601 and 1381 cm⁻¹, respectively. Values of k_s at 120 Wm⁻² are found to be higher than those reported from UV photo-Fenton reactions of GA in bulk aqueous phases containing H₂O₂, Fe²⁺ or Fe³⁺. The implication of our studies on the aging of multicomponent aerosols containing organic matter, transition metals and halide ions due to heterogeneous photochemistry is discussed.