Recent observations suggested that formaldehyde can be incorporated in vegetation at a very high rate. This encouraged our laboratory to develop a methodology for determining tritiated formaldehyde (CHTO) in gaseous effluents containing HTO and HT as dominant species. CHTO being very soluble in water is collected in a solution of carrier formaldehyde. This carrier is necessary for precipitating the formaldehyde derivative of dimedone and collecting it by filtration. The precipitate, which contains the formaldehyde hydrogens, is freed from exchangeable tritium, dried in a oven, and combusted to water for tritium determination. CHTO can thus be separated from HTO with a high efficiency, leading to the possibility of determining accurately 1 Bq of CHTO in as much as 5 × 104 Bq of HTO. The methodology has been applied in preliminary experiments to determine the ratio of CHTO to HTO in effluents from a tritium-handling facility and effluents released from solid miscellaneous wastes. The median of the ratio of CHTO to HTO was 1.2 × 10−3 for the tritium-handling facility (40 samples), and 4.5 × 10−4 for miscellaneous solid wastes (12 samples).