This is an architectural science inquiry in which temperatures were simultaneously observed in buildings with low and high thermal mass at the equator. Four environmental test chambers with different thermal mass levels were monitored under different ceiling types in Nairobi, Kenya, during the warns period between January and March 1997. Walling for two test chambers was natural stone while for the other two was timber paneling. Further to this, roofing for two test chambers was heavy concrete tile while for the other two was lightweight galvanized corrugated iron (GCI) sheets. The effect of thermal mass in lowering the maximum indoor daytime temperatures was evaluated. The loss mass test chambers closely followed outdoor conditions and did not offer any significant thermal storage. All the light-mass test chambers without ceilings recorded small effect on the indoor maxima. However, high thermal mass was very effective in lowering indoor maximum temperatures below the high outdoor maxima. On a hot day in February, when maximum outdoor temperature was over 33 degreesC (91 degreesF), the indoor maximum temperature in high mass building was 25.4 degreesC (77.7 degreesF), which is within the comfort zone. (C) 2003 Elsevier Science B.V. All rights reserved.