Air pollution associated with emissions of fine particulates (PM10 and PM2.5) is becoming acute, especially in the urban environments of Sri Lanka during the last decade. Recent air quality data obtained from the monitoring stations located in Colombo Fort revealed that the average annual ambient PM10 concentrations in Colombo ranged between 72 and 82?µg/m3, exceeding the national ambient air quality PM10 standard of 50?µg/m3 and WHO air quality guideline of 20?µg/m3. This paper presents the chemical compositions of indoor and outdoor PM10 at selected locations in Colombo, Sri Lanka. Twelve-hour indoor/outdoor PM10 were collected simultaneously from six naturally ventilated roadside households from 13 March to 27 April 2010. Chemical compositions were obtained by using thermal optical reflectance method for carbonaceous compounds and ion chromatography method for water-soluble ions. The results show that indoor micro-environments were significantly affected by the infiltration of outdoor air in Sri Lanka, revealing that outdoor emission sources have significant effects on the indoor air quality. This study provides valuable information on chemical profile of particulates in Colombo which will definitely contribute to the formulation of environmental policy, especially on air pollution control in the future.