Large emissions of nitrogen oxides (NOx) are a cause of major environmental damage, and ships account for a large and growing share of these emissions. A problem with the emission standards for NOx adopted by the International Maritime Organisation is that they will apply to new ships only, and the turnover of the fleet is slow. This report assesses potential market-based instruments for reducing emissions from existing vessels and an early introduction of efficient NOx abatement technologies for newly built ships. A rough calculation of the emission reduction potential indicates that application of an emissions charge, as outlined in the report, could cut NOx emissions from ships in the Baltic Sea by around 60 per cent. This would correspond to a reduction of about 270,000 tons in the year 2015.