The effects of the supercritical water environment on the triplet-triplet annihilation of anthracene, a simple, well-characterized reaction that is known to be diffusion controlled in normal liquids, was investigated at temperatures from 375 to 450 degrees C and pressures from 50 to 350 bar. The reaction was found to occur just slightly above the diffusion-control limit, which was estimated from the Stokes-Einstein based Debye equation, when spin statistical factors are taken into account. This is in qualitative agreement with previous studies of diffusion-controlled reactions in lower temperature supercritical fluids (SCFs). Thus, the supercritical water environment is similar to that of lower temperature SCFs for diffusion-controlled reactions of nonpolar hydrocarbon species. There is no evidence of solvent clustering and any influence of solute/solute interactions is small. Finally, the mechanism of the reaction appears to be the same as in nonpolar liquid solvents.