Results are presented from a study of south facing roof apertures on two buildings from the U. S. Department of Energy's Non-Residential Experimental Passive Buildings Program. This study is part of a broad effort to evaluate the energy, functional and economic performance of buildings. The work reported here supplemented the basic building data with illumination data from physical models and from the buildings themselves, and with additional observations and measurements. Energy analyses were performed in order to examine the daylighting systems incorporated into the buildings, especially as they relate to (a) interaction between heating, cooling and lighting requirements; (b) integration of the electric lighting and daylighting systems and (c) functional effectiveness and occupant interaction with the daylighting system.