Barium partitioning between alkali feldspar and a natural trachyte liquid, enriched with barium, has been determined as a function of pressure and temperature from 10 to 25 kb and 900Ll100 ~ C. Both long duration experiments and a "re-equilibration" experiment suggest close approach to equilibrium. Partition coefficients (DBa) decrease as both temperature and pressure increase (e.g., DBa changes from 8.71 at 10kb, 900~ to 1.48 at 25kb, 1100 ~ C). Water activity also controls the barium partitioning with a marked decrease in n,f/llq for addition of less ~B a than 0.8 wt% H20, but with no apparent additional effect for higher water contents in the bulk composition (e.g., from 0.8-4.2 wt% H20). The composition of alkali feldspar /laf/liq but the data obtained also has a significant effect on ~B. , do not allow derivation of a complete D-Or relationship. These new data suggest that Henry's Law is obeyed for most of the barium concentrations examined, and the limit of Henry's Law behaviour for barium in alkali feldspar is as high as 6 wt% BaO in alkali feldspar and 1.2 wt% BaO in the melt, similar to the results of Long (1978). The experimental results broadly overlap with natural data for DBa, determined from coexisting alkali feldspar phenocrysts and glass (or groundmass).