Controversy over EU-wide biofuel policy resonated within the UK, fuelling policy disagreements among UK public authorities. They disagreed over how to protect a space for future second-generation biofuels, which were expected to overcome harm from first-generation biofuels. The UK government defended rising targets for available biofuels as a necessary stimulus for 'industry to help fulfil the UK's EU obligations and eventually develop second-generation biofuels. By contrast, Parliamentary Select Committees opposed biofuel targets on grounds that these would instead lock-in first-generation biofuels, thus delaying or pre-empting second-generation biofuels. Those disagreements can be explained by different institutional responsibilities and reputational stakes towards 'promise-requirement cycles', whereby techno-optimistic promises generate future requirements for the actors involved. The UK government's stance illustrates a 'policy-promise lock-in', a dilemma whereby promised support is a requirement for credibility towards technology innovators and thus technoscientific development but may delay the redirection of support from incumbent to preferable emerging technologies. Thus the sociology of expectations - previously applied to technological expectations from technology innovators - can be extended to analyse public authorities. (C) 2013 Published by Elsevier Ltd.