EXPOLIS was a large-scale population-based study of urban adult exposures to multiple pollutants, and was conducted between 1996 and 2000 in six European cities. Measurements made using standardised protocols in Athens (Greece), Basel (Switzerland), Helsinki (Finland), Milan (Italy), Oxford (UK), and Prague (Czech Republic), allow similarities and differences between contrasting European regions, climates and populations to be identified. Two consecutive days of home indoor and home outdoor measurements of fine particulate matter (PM2.5), black smoke (BS), and nitrogen dioxide (NO2) were carried out at the homes of adult participants on different dates and seasons during the sampling period. Regression models with interactions searched by all-possible subset method were used to assess the city effects and the determinants of home indoor PM2.5 (adj R2=0.60, n=413), BS (adj R2=0.79, n=382) and NO2 (adj R2=0.67, n=302) levels. Both bi-directional (positive and negative signs of associations) and unidirectional (consistently either positive or negative sign of associations) city effects on different determinants in each indoor model were shown. Smoking, gas-stove usage, outdoor temperature, and wind speed were the common determinants in all three indoor models. Other determinants, including the presence of wooden material, heating, and being located in suburb area, were also identified. They were likely linked to cultural and socio-economic factors.