Trichlorofluoromethane (CCl₃F; CFC‐11) and dichlorodifluoromethane (CCl₂F₂; CFC‐12) have been widely used as tracers of oceanic circulation and mixing on decadal timescales. In order to estimate their transfer rate across the air‐sea interface, liquid‐phase diffusion coefficients are needed In this study the diffusivities of CFC‐11 and CFC‐12 in pure water were measured over the temperature range 0.6–30°C. Diffusivities of CFC‐11 in pure water ranged from (5.24±0.25)×10⁻⁶ cm²S⁻¹ at 0.6°C to (1.13±0.05)×10⁻⁵ cm²S⁻¹ at 30.3°C and a fit to the data yielded the equation D_CFC‐11 = 0.015 exp (−18.1/RT), where R is the universal gas constant in kJ mol⁻¹ K⁻¹ and T is the temperature in Kelvin. Diffusivities of CFC‐12 in pure water ranged from (5.38±0.22)×10⁻⁶ cm² s⁻¹ at 0.6°C to (1.26±0.05)×10⁻⁵ cm² s⁻¹ at 30.3°C and the temperature dependence can be expressed as D_CFC‐12= 0.036 exp (−20.1/RT). The estimated uncertainty in both equations is <3%. Experiments were also carried out in seawater for each compound. For CFC‐11 the diffusivity in seawater was not significantly different from that in pure water. However, the diffusivity of CFC‐12 in seawater was found to be 7.2±3.0% lower than that in pure water. Schmidt numbers for both CFC‐11 and CFC‐12 in pure water and seawater were estimated from the data.

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