Oxygen O-18 tracer diffusion in Czoehralski-grown mullite single crystals is investigated along [010] and [001]. Oxygen diffusion coefficients range between similar to5 x 10(-20) m(2)/s (1250 degreesC) and similar to9 x 10(-18) m(2)/s (1525 degreesC). The data does not show any significant anisotropy. The values of the activation enthalpy (4.5 eV) and of the activation entropy ((3.4 +/- 1.6)k(B), where k(B) is the Boltzmann constant) suggest that the atomic transport occurs via thermally activated vacancies.