As part of the Swedish-Danish agreement of mutual aid in submarine rescue, the exercise Royal Crown was carried out in September 2000. The mission was to carry out trials with the Danish Distressed Submarine Ventilation and Decompression System (DSVDS) and to demonstrate the ability to co-operate in rescue of submariners from a distressed submarine. The Defence Research Establishment (FOA), Human Sciences Division, Department of Defence Medicine was requested to carry out air quality analysis on board HDMS Nordkaperen (NOR) during trials with the Ventilation and Decompression System. Oxygen (O2), carbon dioxide (CO2), hydrogen (H2), and carbon monoxide (CO) concentrations along with pressure, temperature and humidity were continously monitored. The aerosol concentrations and the total concentration of volatile organic compounds (TVOC) in the submarine atmosphere were also analysed. The CO2 concentration during the DSVDS test did not exceed 1.1% and the O2 level never fell below 20%. The pressure span was 127 to 136 kPa with an average of 131 kPa. The highest aerosol concentration detected was 3.6 mg/m3. Increased CO and H2 concentrations were not detected. The VOC samples all showed patterns typical of fuel vapour and the total concentrations (TVOC) were 30-35 mg/m3 toluene equivalents. Due to large variation in physical activity on board and to the fact rhat carbon dioxide scrubbing was running during the first day it is impossible to state to what extent the ventilation contributed to the steady state level of CO2 (1.03%). However, the carbon dioxide level never exceeded the theoretical steady state level of 1.23%. There are two ways to reach a lower steady state level and that is to increase the ventilation and/or to reduce the physical activity of the crew in order to reduce the CO2 production.