This paper examines the statistical properties of occupancy in single person offices of a large office building in San Francisco. A probabilistic model to predict and simulate occupancy in single person offices is proposed. It is found that vacancy intervals are exponentially distributed and that the coefficient of the exponential distribution for a single office could be treated as a constant over the day. Occupancy intervals are more complex than vacancy intervals. The distribution of occupancy intervals is time varying. Variations among different offices are examined. The implications of the findings on thermal and air quality control are discussed. (C) 2004 Elsevier B.V. All rights reserved.