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　mu　g/m(3),　exceeding　the　national　ambient　air　quality　PM10　standard　of　50　mu　g/m(3)　and　WHO　air　quality　guideline　of　20　mu　g/m(3).　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.