In this randomized-controlled, cross-over trial the investigators test the effect of long-term oxygen therapy (> 16h/day) given for 5 weeks with a wash-out period between phases of 2 weeks on exercise performance and quality of life and many physiological parameters in patients with pulmonary arterial and chronic thromboembolic pulmonary hypertension.

Inclusion criteria:

- Adult patients with established diagnosis of pulmonary arterial or chronic thromboembolic pulmonary hypertension (class I and IV according to the latest WHO classification scheme of Dana Point 2008) on optimized medical therapy and in stable condition for at least 4 weeks, who desaturate in the 6 minute walking test by =4% to values <92%.

Exclusion criteria:

• Patients in unstable conditions requiring frequent therapeutic adaption
• pregnant women
• patients with pulmonary venous hypertension due to left heart diseases
• patients with relevant concomitant lung disease and severe daytime hypoxemia