Mineralization of organic matter in environmental samples is frequently quantified by trapping ¹⁴CO₂ released from the degradation of ¹⁴C-labelled compounds. However, when ¹⁴CO₂ trapped in NaOH is quantified by liquid scintillation counting, ¹⁴C-activity can be gradually lost from the scintillation vials. When combining different molarities and volumes of NaOH and five commercial scintillation cocktails, we observed in some mixtures a rapid loss in ¹⁴C-activity of up to 10% after 0.5 h and 60% after 96 h, while other mixtures showed no loss of ¹⁴C-activity for at least 96 h. The ¹⁴C-activity loss depended on molarity, volume, and CO₂ saturation of NaOH, as well as the scintillation cocktail used and the mixing ratio of the two components. We show that the decline of ¹⁴C-activity may be caused by the loss of ¹⁴CO₂ from the scintillation vials. The loss of ¹⁴C-activity must be prevented and therefore we provide recommendations how to accurately quantify ¹⁴C-activity of ¹⁴CO₂ trapped in NaOH using liquid scintillation counting.