The Commission of the European Communities Directorate-General for Science, Research and Development (DG XII) has recently published an overview of research and results within the 4th multiannual RTD programme in the field of the environment (1986-1990), entitled "Air Quality. Analysis, Sources, Transport, Transformation and Deposition of Pollutants". The 4th environmental research and development programme began in 1986. However, research projects on air quality started only in early 1988, some of them extending to 1992. In this research area, thirteen implemented projects were related to the analysis, sources, transport, transformation and deposition of pollutants, which were topics covered within the concerted action COST 611 (Physico-chemical Behaviour of Atmospheric Pollutants). Four of these projects were also included in EUREKA (the LACTOZ and HALIPP sub-projects of EUROTRAC dealing with laboratory research on gas phase and heterogeneous and liquid phase processes). The aim of this research was to improve the knowledge of tropospheric chemistry by addressing the major problems related, essentially but not exclusively, to acid deposition and ozone and photooxidant pollution. More than thirty teams from nine Member States were involved. The present publication describes the main achievements of each project, preceded by a summary. The report also contains a significant list of papers relevant to the projects and published in the scientific literature, including the proceedings of the 1989 Symposium of the COST 611 Action.