A numerical model for scanning electrochemical microscopy (SECM) double potential step chronoamperometry (DPSC) has been developed and examined experimentally. The concept of this new mode of SECM is to generate a reactant in an initial potential step at a tip ultramicroelectrode (UME) positioned close to a target interface. The electrogenerated species diffuses from the tip to the interface, where it may bl : involved in a chemical process. The reactant is subsequently collected by electrolysis in a second potential step, and the form of the corresponding current-time curve provides :information on the nature of the interaction between the initial tip-generated species and the interface. If the species is consumed in an irreversible interfacial process, the current flow during the second potential step is less than when the interface is inert with respect to the species of interest. The theoretical predictions are first examined with DPSC studies on the electrogeneration and collection of ferricyanide ions from aqueous ferrocyanide solutions, at a tip positioned close to aqueous/glass, aqueous/1,2-dichloroethane (DCE), and aqueous/air interfaces, as mod-l examples of inert liquid/solid, liquid/liquid, and liquid/gas interfaces. The case of an active interfacial process is illustrated through studies of the electrogeneration and collection of Br-2, from aqueous sulfuric acid solutions of potassium bromide, at a tip positioned close to aqueous/DCE and aqueous/air interfaces. The transfer of Br-2 across these interfaces is found to be irreversible and effectively diffusion-controlled on the SECM rime scale, putting a lower limit on the interfacial transfer rate constant of 0.5 cm s(-1). The experiments carried out at aqueous/air interfaces represent the first demonstration that SECM can be used to probe liquid/gas interfaces, thereby further diversifying the range of novel environments that can be studied with this instrument.