The identification of particulate matter sources is an important step in the development ofindoor air quality control strategies. Advanced modelling techniques, termed as receptormodelling, are used to identify the presence, determine the sources of aerosols and mostimportantly quantify the source contributions. This study presents a source apportionmentanalysis, supported by targeted experimental campaign, to identify the main pollutionsources in an office indoor environment. For the purpose of the experimental campaign,continuous monitoring of particle (0.3 - andgt; 20.0 μm in aerodynamic diameter) numberconcentrations took place during the period between 16th and 27th July 2007 in two officeenvironments, in Athens, Greece. A portable dust monitor (GRIMM model 1.108) was usedto measure in real-time the particles size distribution in 16 different size channels. Twoadjacent offices -with similar characteristics except for the presence of smokers- were used.Additionally, the contribution of other possible sources (e.g. photocopy machine, airconditioner, penetration and outdoor environment) was investigated. The indoortemperature, relative humidity and light intensity levels were also recorded in a continuousbasis. Finally, all activities were daily recorded in detail on a logbook. The positive matrixfactorization (PMF) method has been applied on the data collected during the experimentalcampaign for source apportionment analysis. The output of the analysis has been relatedwith source information provided by the occupants and other source profiles bybibliography. This study aims to elicit indoor source characteristics based on particles sizedistribution data, especially in situations where the indoor sources are unknown beforehand.