The measured radiogenic 40Ar loss from sized biotite (56% annite) samples following isothermalhydrothermal treatment have provided model diffusion coefficients in the temperature interval 600°C to 750°C, calculated on the assumption that Ar transport proceeds parallel to cleavage. These data yield an array on an Arrhenius plot with a slope corresponding to an activation energy 47.0 ± 2 kcal-mol -1 and a frequency factor of 0.077 +0.21-0.06 cm 2-sec -1. Together with previous diffusion data for micas in the annitephlogopite series, our results indicate a strong compositional effect, with increasing Fe/Mg ratio corresponding to an increase in diffusivity. An effective diffusion radius of about 150 μm for biotite is inferred from the experimental data which compares favorably with that estimated from geological studies. A pressure effect on activation energy corresponding to an activation volume of about 14 cm 3-mol -1 is observed. These data yield closure temperature estimates for this biotite composition cooling at rates of 100°C-Ma -1, 10°C-Ma -1 and 1°C-Ma -1 of 345°C, 310°C and 280°C, respectively. 40Ar /39Ar age-spectrum analysis of a hydrothermally treated biotite yields a complex release pattern casting doubt on the general usefulness of such measurements for geochronological purposes.