While the environmental and economic dimensions of sustainability have received plenty of attention in biofuel policies and assessment, only recently has the social 'pillar' gained increasing weight, demonstrated e.g. by debates over the dilemmas such as food vs. fuel and large vs. small-scale biofuel production. This paper calls for greater attention to power relations when assessing biofuel policies. A centre-periphery framework is applied for examining power relations in the Brazilian sugar and alcohol sector since the launching of the country's transport biofuel programme, Proalcool, in 1975. Particular attention is paid to the country's poor Northeast, today responsible for a small fraction of bioethanol production in Brazil, but highly dependent on its sugar and alcohol sector for employment and economic output. The analysis demonstrates the pervasive role of unequal and rigid power relations in shaping Proalcool's social impacts. The small sugar elite in the Northeast has increased its power and protected itself against market instability by diversifying its activity, while the benefits have failed to trickle down to the poorest in the sugarcane zone in the Northeast coast. The recent and on-going entry of new players in the Brazilian biofuel scene may provide opportunities of breaking the entrenched power structures and creating space for more pro-poor policies, notably through international sustainability certification. Any such schemes must, however, be designed carefully to avoid capture by the regional elites, and counteract the present tendencies towards further concentration of power - a danger particularly acute in the development of advanced next generation biofuels. (C) 2009 Elsevier Ltd. All rights reserved.