The mechanisms and kinetics of the photoreactions taking place at the n-MoSe2/I- interface have been studied on both macroscopic and microscopic scales by means of photocurrent transients, intensity-modulated photocurrent spectroscopy, and the scanning microscope for semiconductors characterization (SMSC) method. SMSC images show that stepped regions can give higher photocurrents than the van der Waals surface of the crystal. Three different effects have been found to be responsible for the complex transient photoresponse of the system : (1) the recombination of photogenerated electron-hole pairs at surface defects, (2) the electrochemical reduction of photogenerated I-3(-) ions at surface steps, and (3) the increase in the efficiency of interfacial hole transfer associated with chemisorption of I-3(-) at step sites. A comparison of local and global photoresponses shows that the adsorption of photogenerated triiodide ions is responsible for an "autocatalytic" effect on the photocurrent response.