Isobaric counterdiffusion is a technique used in diving based on the idea that different inert gases have different speeds at which they are absorbed and released from the body. During a dive, divers switch the initial mix breathed in from one inert gas to another, allowing one gas to diffuse out of the body while another is absorbed without changing pressure. This series contains material related to Lambertsen's experiments with isobaric counterdiffusion including data logs, experimental results, correspondence, experimental tables, slides, news clippings, NIH grant renewal proposals, and reports. Materials range in date from 1973 to 2001.