Experiments investigating the isotopic fractionation in the formation of H<sub>2</sub> by the photolysis of CH<sub>2</sub>O under tropospheric conditions are reported and discussed. The deuterium (D) depletion in the H<sub>2</sub> produced is 500(±20)‰ with respect to the parent CH<sub>2</sub>O. We also observed that complete photolysis of CH<sub>2</sub>O under atmospheric conditions produces H<sub>2</sub> that has virtually the same isotope ratio as that of the parent CH<sub>2</sub>O. These findings imply that there must be a very strong concomitant isotopic enrichment in the radical channel (CH<sub>2</sub>O+<i>h</i>ν → CHO+H) as compared to the molecular channel (CH<sub>2</sub>O+<i>h</i>ν → H<sub>2</sub>+CO) of the photolysis of CH<sub>2</sub>O in order to balance the relatively small isotopic fractionation in the competing reaction of CH<sub>2</sub>O with OH. Using a 1-box photochemistry model we calculated the isotopic fractionation factor for the radical channel to be 0.22(±0.08), which is equivalent to a 780(±80)‰ enrichment in D of the remaining CH<sub>2</sub>O. When CH<sub>2</sub>O is in photochemical steady state, the isotope ratio of the H<sub>2</sub> produced is determined not only by the isotopic fractionation occurring during the photolytical production of H<sub>2</sub> (α<sub><i>m</i></sub>) but also by overall fractionation for the removal processes of CH<sub>2</sub>O (α<sub><i>f</i></sub>), and is represented by the ratio of α<sub><i>m</i></sub>/α<sub><i>f</i></sub>. Applying the isotopic fractionation factors relevant to CH<sub>2</sub>O photolysis obtained in the present study to the troposphere, the ratio of α<sub><i>m</i></sub>/α<sub><i>f</i></sub> varies from ~0.8 to ~1.2 depending on the fraction of CH<sub>2</sub>O that reacts with OH and that produces H<sub>2</sub>. This range of α<sub><i>m</i></sub>/α<sub><i>f</i></sub> can render the H<sub>2</sub> produced from the photochemical oxidation of CH<sub>4</sub> to be enriched in D (with respect to the original CH<sub>4</sub>) by the factor of 1.2–1.3 as anticipated in the literature.