An intravascular gas exchange device for the treatment of respiratory failure consisted of a multitude of blind-ended hollow fibers glued in a pine-needle arrangement to a central gas supply catheter. It has previously been shown that gas desorption rates can be significantly enhanced by cycling gas pressure between a hypobaric level of 130 and an ambient level of 775 Torr. In this study, influences of the cycling frequency (f) and the cycle fraction during which hypobaric pressure is applied (Θ) were investigated. Rates of O2 desorption from O2-saturated water and CO2 desorption from CO2-saturated water into a manifold containing 198 fibers, 380 μm in diameter, were measured over a range of f from 0.2 to 1.0 Hz, Θ from 0.1 to 0.8, and fiber lengths from 4 to 16 cm. Relative to operation at ambient pressure, pressure cycling increased O2 transfer 3–4 times and CO2 transfer 4–6 times when the water flowed over the fiber manifold at 2.3 ℓ/min. Transfer rates were relatively insensitive to f and Θ with 80–90% of maximum enhancement obtained when Θ was as low as 0.2. Transfer rates increased continuously with fiber length, implying that pressure cycling reduced the intra-fiber resistance to gas diffusion. A mathematical diffusion model which utilized only two adjustable parameters, a mass transfer coefficient for O2 and for CO2, simulated the trends exhibited by desorption data.