This paper analyzes differences in socioeconomic inequality -in particular, marginalitybetween the Mexican and U.S. borderlands. The analysis is based on a theoretical model of binational border inequality. Marginality indicators, based on the COPLAMAR studies, reflect lack of benefits from national growth, socioeconomic development, and wealth for much of Mexico. Analysis of variance reveals differences for marginality indicators between border regions, non-border regions, and the Mexico City metropolitan zone. Results show that marginality varies greatly within Mexico, varies slightly within the United States, and varies most substantially between Mexican and U.S. regions. The results confirm the postulated role of the Mexican borderlands as a buffer region in inequality levels between the two nations.