Over the past decade, the cultivation of rubber trees has expanded rapidly throughout the Mekong region to non-traditional rubber growing areas of Laos and Myanmar. Prompted by rising prices from 1990 to 2010 and government agro-industrialization policies, farmers and investors have rushed to plant the new boom crop. A latex price crash in 2011, however, has made it more challenging for small-scale producers to earn an income, leading to uneven social-ecological transformations and economic consequences. Several proposals have been made to address these challenges by transforming rubber into a more economically, socially, and environmentally sustainable crop. In this paper, which emerged from one such project to investigate the potential for "green" rubber, we argue that the sustainability of rubber is a challenging and elusive prospect – particularly in resource frontier contexts like Laos and Myanmar. Concepts like "sustainability" or "green" production are vague and malleable. They can be imbued with a variety of contradictory meanings, which often do not address the most socially and environmentally problematic aspects of cash crop expansion. Sustainable rubber, if rigorously and specifically defined, would be exceedingly difficult to reach in both countries, due to the ways in which political-economic and governance factors interact with the biophysical and social characteristics of the crop. Instead, we recommend using sustainability as a political tool for highlighting the most harmful socio-environmental impacts of rubber and generating debate concerning the best ways to address these, thus limiting unsustainable practices.