The activity coefficient and the diffusion coefficient of boron in single-crystalline silicon have been determined over a temperature range of 1064-1274 K. Boron vapor with a controlled activity was equilibrated with silicon surface in a quarts capsule. Transferred boron then diffused into the silicon. The activity coefficient of boron was determined from the boron concentration at the silicon surface assuming a local equilibrium between a boron-containing gas and the surface. The temperature dependence of the activity coefficient at infinite dilution relative to pure solid boron is expressed as ln gamma degrees(B) = 1.36 - 6300/T. From the obtained gamma degrees(B) the standard Gibbs energy of formation of SiB3(s) is evaluated as Delta G degrees(SiB3)/J mol(-1) = -250000 + 26.3 T. The diffusion coefficient of boron is determined from the concentration profile as D-B/m(2).s(-1) = 4.9 x 10(-7) exp[-290000 J/RT] = 4.9 x 10(-7) exp[-3.01 eV/kT] where R is the gas constant, 8.31 J/K mol, and k is the Boltzmann's constant, 8.62 x 10(-5) eV.