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Up to one tenth of the carbon dioxide (CO₂) emissions from inland waters worldwide are directly induced by the photochemical mineralization of dissolved organic matter (DOM). The photochemical production of dissolved inorganic carbon (DIC) per photon absorbed by chromophoric DOM (CDOM) decreases exponentially with increasing irradiance wavelength, and is commonly described by an “apparent quantum yield” (AQY) spectrum. Although an essential model parameter to simulate photochemical mineralization the AQY remains poorly constrained. Here, the AQY of photochemical DIC production for 25 lakes located in boreal, polar, temperate, and tropical areas, including four saline lagoons, was measured. The wavelength‐integrated AQY (300–500 nm; mol DIC mol CDOM‐absorbed photons⁻¹) ranged from 0.05 in an Antarctic lake to 0.61 in a humic boreal lake, averaging 0.24 ± 0.03 SE. AQY was …