This study investigates the enhancement of the oxygen diffusion rate at the cathode of a proton exchange membrane fuel cell (PEMFC) due to pure oscillating flow. A unit cell of PEMFC using hydrogen fuel and oscillating air was tested. The experimental results show that the non-dimensional effective diffusivity varies linearly with the square of the Womersley number, when the Womersley number is close to unity. The non-dimensional effective diffusivity varies linearly with the Womersley number itself when the Womersley number is much larger than unity. Similar trend has been confirmed from the theoretical approach. Under the experimental conditions in this study, the reaction rate of oxygen increased linearly with respect to the sweep distance. The experimental results showed that a power density of 115.4 mW/cm(2) was obtained from the unit cell with oscillating flow, which is comparable to that obtained with forced flow. Therefore, an oscillating flow is found to be able to increase the concentration of the oxygen in the channel of PEMFCs, and consequently enhances mass-transfer, similarly to the use of forced flow using blowers or compressors. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.