This research aims to quantify the effects of car sharing on car ownership, car use and CO2 emissions. The results are based on a survey amongst 363 car sharing respondents in the Netherlands. We found over 30% less car ownership amongst car sharers and they drove 15% to 20% fewer car kilometres than prior to car sharing. The shared cars mostly replace a second or third car. Due to reduced car ownership and car use, car sharers emit between 240 and 390 fewer kilograms of CO2 per person, per year. This is between 13% and 18% of the CO2 emissions related to car ownership and car use.