During early saturation diving experiments, subjects in a high pressure heliox atmosphere who temporarily breathed nitrogen-rich gas mysteriously developed skin rash, itching, and vertigo. These signs and symptoms were characteristic of skin bends and inner ear decompression sickness, but no decompression had occurred. This isobaric counterdiffusion disease is caused by bubble formation, resulting not from decompression, but from supersaturation that can develop with unequal tissue uptake and washout of different inert gases. The skin manifestations result from diffusion of helium from the atmosphere across the skin, and this mechanism is of little relevance to technical divers whose skin is not exposed to helium. However, inner ear injury is of particular concern to technical divers. Inner ear symptoms such as vertigo and vomiting can be life-threatening if they occur in the water, and inner ear injury can be difficult to treat. Inner ear injury can result from sequential breathing of helium-rich and then nitrogen-rich mixtures. Such gas switching is common during decompression from deep dives, where it is believed it might contribute to development of inner ear decompression sickness. However, recent work suggests helium-to-nitrogen gas switches play only a minor role in inner ear decompression sickness. This talk will explore the mechanism of isobaric counterdiffusion disease and in particular how helium-to-nitrogen gas switches may or may not contribute to injury in technical diving.