Recent studies have revealed the crucial role played by the macro corrosion cell (potential coupling between the inside and outside of a cavity) in crevice and pitting corrosion. It was found that acidification and the existence of chloride ions in the local cell are not the sole and necessary conditions for localized corrosion to occur, and that their accelerating effects on crevice corrosion and pit growth can be explained within the framework of the macro cell (the IR voltage mechanism). Upon analysis of the results of the experiments, quantitative modeling, and the literature, a new characteristic parameter - the critical distance into the crevice, dc - has been suggested for indexing the crevice corrosion resistance of a material under specified conditions. The advantages of using dc as the index of the crevice corrosion resistance are: (1) it may be obtained through experiment and may also be estimated through computational approaches; (2) it has a distinct and straightforward physical meaning; (3) it may be employed in engineering design and (4) it is a single parameter which can reflect the integrated influence of several factors known to affect the crevice corrosion resistance of a material from past practical experience and research work. Preliminary work has shown good agreement between the measured and the computed values of dc. The experimental technique and the principle of the mathematical approach to obtain dc are described.