AbstractElectroconvulsive therapy (ECT) is an effective treatment for depressive disorders. In certain cases, ECT-associated anaesthesia can be improved by the use of ketofol (i.e., S-ketamine + propofol). We aimed to evaluate the empirical mixing ratio of ketofol in these cases for better clinical implementation. We retrospectively investigated n = 52 patients who received 919 ECT sessions with S-ketamine plus propofol as anaesthetic agents. Several anaesthesia and ECT-related parameters including doses of S-ketamine and propofol were analysed. The mean empirically determined S-ketamine/propofol ratio was 1.38 (SD ± 0.57) for 919 individual ECT sessions and 1.52 (SD ± 0.62) for 52 patients, respectively. The mean relative dose was 0.72 (± 0.18) mg/kg S-ketamine and 0.54 (± 0.21) mg/kg propofol. Higher propofol dose was associated with poorer seizure quality. Seizure quality and time in recovery room were significantly influenced by age. Ketofol could be an option to exploit the advantageous qualities of S-ketamine and propofol, if both doses are reduced compared with single use of S-ketamine or propofol. Patients with poor seizure quality may benefit from lower propofol doses, which are applicable by the addition of ketamine. An empirically determined mixing ratio in favour of ketamine turned out to be preferable in a clinical setting. Recovery time was primarily prolonged by higher age rather than by ketamine dose, which had previously often been associated with a prolonged monitoring time in the recovery room. These new findings could improve electroconvulsive therapy and should be replicated in a prospective manner.