Gas diffusivity in snow and firn is a basic transport parameter that is important for snow-air exchange processes and for ice core interpretation of past atmospheric composition. Firn is snow that is more than one year old, and it exists over many thousands of kilometers of the polar ice sheets, comprising approximately the upper 60-120 m of the ice sheet. Because use of firn air for climate and environmental investigations is a relatively new science, and because of the difficulties in working with firn as a temperature-dependent porous material, few direct measurements of the gas diffusivity through the firn column have been made. This paper describes the development, testing, validation, and initial use of a diffusion chamber specifically designed to measure inert gas diffusivity on firn cores. A brief review of previous methods for determining gas diffusivity for various porous media is discussed. The new technique and apparatus are described. Results of validation of the technique on multiple sizes of glass beads are presented, including comparison to experimental and theoretical results from the literature. After this verification, initial results obtained from using the technique to measure the SF6 gas diffusivity of firn samples from Summit, Greenland are presented; results ranged from 0.005 cm(2)/s to 0.033 cm2/s. This set of measurements is compared to the diffusivity of other natural porous media and also to the few previous measurements of gas diffusivity of firn. The improved technique offers a more direct measurement of the transport parameter than other techniques previously used, and offers good potential for future investigations aimed at understanding processes of firnification and gas trapping for ice core climate change research. (C) 2013 Elsevier Ltd. All rights reserved.