Marine oil transportation in the Baltic Sea has increased significantly during the past decade. This may pose a significant threat to the environment, partly due to the increased risk of accidents and partly because of deliberate flushing of oil tanks at sea and other diffuse emissions. The problem is examined from a Swedish point of view, emphasizing possible ways of reducing environmental risks associated with marine oil transportation in which cooperative solutions will be required to solve current collective action dilemmas. It is argued that since international conventions tend to suffer from lowest-common-denominator (LCD) effects whereby the least interested actors often set the level of ambition, pro-active countries may benefit from adopting dual strategies where unilateral initiatives and convention-based cooperation are made part of an integrated approach. Countries such as Sweden that are especially vulnerable to ecological threats from marine oil transportation may thus have strong incentives to provide targeted support to less exposed countries. It is concluded that unilateral and sub-regional initiatives may serve an important objective in complementing international conventions and thereby reduce negative effects from Lcd outcomes.