Daylight plays a major role in making atrium spaces appealing to building users. This appeal is also evident in the spaces adjoining the atrium. A key component of atrium daylighting, especially in tall atria, is the daylight reflected on to the atrium walls. This study examines the impacts of well geometries and different diffuse reflectance distributions of walls on variations of vertical internally reflected daylight across the walls of atria. The square atrium models used in the study have a very wide range of well index (WI) values, ranging from 0.25 (shallow atrium) to 2.0 (deep atrium). In addition, two well wall reflectance distribution patterns were used, namely horizontal and vertical bands. Validated by scale model measurements, the lighting package Radiance was used to predict vertical internally reflected daylight levels. It was found that different reflectance distributions do have an impact on the vertical internally reflected components across the wall of the square atrium models under overcast sky conditions. The impact relates to the orientation of the band with different reflectance distributions on the wall. For central wall areas at locations greater than one third of atrium height the horizontal distributions of the reflectances significantly affect the vertical internally reflected components. For the wall area near the atrium corner the effects are quite complicated, being decided by both the horizontal reflectance distributions and vertical positions. For an atrium with a height more than 0.5 its well width the reflectance distribution influence tends to increase with increasing well index. The vertical distributions of the reflectance, however, do not substantially affect the vertical daylight levels in atria except for some special reflectance distribution patterns. (C) 2011 Elsevier Ltd. All rights reserved.