Background: Dyscarbia is common in out-of-hospital cardiac arrest (OHCA) patients and its association to neurological outcome is undetermined. Methods: This is an exploratory post-hoc substudy of the Target Temperature Management (TTM) trial, including resuscitated OHCA patients, investigating the association between serial measurements of arterial partial carbon dioxide pressure (PaCO₂) and neurological outcome at 6months, defined by the Cerebral Performance Category (CPC) scale, dichotomized to good outcome (CPC 1 and 2) and poor outcome (CPC 3-5). The effects of hypercapnia and hypocapnia, and the time-weighted mean PaCO₂ and absolute PaCO₂ difference were analyzed. Additionally, the association between mild hypercapnia (6.0-7.30kPa) and neurological outcome, its interaction with target temperature (33°C and 36°C), and the association between PaCO₂ and peak serum-Tau were evaluated. Results: Of the 939 patients in the TTM trial, 869 were eligible for analysis. Ninety-six percent of patients were exposed to hypocapnia or hypercapnia. None of the analyses indicated a statistical significant association between PaCO₂ and neurological outcome (P=0.13-0.96). Mild hypercapnia was not associated with neurological outcome (P=0.78) and there was no statistically significant interaction with target temperature (P _interaction=0.95). There was no association between PaCO₂ and peak serum-Tau levels 48 or 72h after return of spontaneous circulation (ROSC). Conclusions: Dyscarbia is common after ROSC. No statistically significant association between PaCO₂ in the post-cardiac arrest phase and neurological outcome at 6 months after cardiac arrest was detected. There was no significant interaction between mild hypercapnia and temperature in relation to neurological outcome.