Scanning electrochemical microscopy (SECM) has emerged as a powerful technique for inducing and monitoring molecular transfer processes across water/air and liquid/liquid interfaces. At the same time, the Langmuir trough technique is a well established method for controlling the lateral pressure of molecular films of amphiphilic molecules at interfaces. A combination of both methods allows the investigation of the permeability of monolayers in a defined state. A brief introduction of the SECM technique and the experimental set-up is presented. The application of the combined SECM-Langmuir trough technique to measure passive diffusion of small molecules (O-2 and Br-2) across phospholipid monolayers is then reviewed. Phospholipid monolayers at liquid/liquid and liquid/air interfaces serve as simple biomimetic models for biomembranes and the results of the combined SECM-Langmuir trough measurements have implications for understanding passive diffusion across cellular membranes.