Abstract Background Differences in physiology of ARDS have been described between COVID-19 and non-COVID-19 patients. This study aimed to compare initial values and longitudinal changes in respiratory system compliance ( C RS ), oxygenation parameters and ventilatory ratio (VR) in patients with COVID-19 and non-COVID-19 pulmonary ARDS matched on oxygenation. Methods 135 patients with COVID-19 ARDS from two centers were included in a physiological study; 767 non-COVID-19 ARDS from a clinical trial were used for the purpose of at least 1:2 matching. A propensity-matching was based on age, severity score, oxygenation, positive end-expiratory pressure (PEEP) and pulmonary cause of ARDS and allowed to include 112 COVID-19 and 198 non-COVID pulmonary ARDS. Results The two groups were similar on initial oxygenation. COVID-19 patients had a higher body mass index, higher C RS at day 1 (median [IQR], 35 [28–44] vs 32 [26–38] ml cmH 2 O −1 , p = 0.037). At day 1, C RS was correlated with oxygenation only in non-COVID-19 patients; 61.6% and 68.2% of COVID-19 and non-COVID-19 pulmonary ARDS were still ventilated at day 7 ( p = 0.241). Oxygenation became lower in COVID-19 than in non-COVID-19 patients at days 3 and 7, while C RS became similar. VR was lower at day 1 in COVID-19 than in non-COVID-19 patients but increased from day 1 to 7 only in COVID-19 patients. VR was higher at days 1, 3 and 7 in the COVID-19 patients ventilated using heat and moisture exchangers compared to heated humidifiers. After adjustment on PaO 2 /FiO 2 , PEEP and humidification device, C RS and VR were found not different between COVID-19 and non-COVID-19 patients at day 7. Day-28 mortality did not differ between COVID-19 and non-COVID-19 patients (25.9% and 23.7%, respectively, p = 0.666). Conclusions For a similar initial oxygenation, COVID-19 ARDS initially differs from classical ARDS by a higher C RS , dissociated from oxygenation. C RS become similar for patients remaining on mechanical ventilation during the first week of evolution, but oxygenation becomes lower in COVID-19 patients. Trial registration : clinicaltrials.gov NCT04385004