MONICAIR --MONItoring & Control of Air quality in Individual Rooms-- is a pre-competitive field research project of a broad consortium of Dutch ventilation unit manufacturers and research institutes, supported by the Dutch government. The aim is to investigate the indoor air quality (IAQ) performance and energy characteristics of 9 different mechanical ventilation solutions in dwellings that meet strict air-tightness standards and comply with current building regulations. Over a full year 62 residential dwellings were monitored, with in each habitable room sensors for occupancy, CO2, relative humidity and air temperature. Energy consumption of the mechanical ventilation units and space heating boilers were also continuously monitored. The ventilation solutions included mechanical exhaust ventilation (MEV) systems (mechanical exhaust in the wet rooms, and with natural air supply in habitable rooms), systems with mechanical exhaust in all rooms, as well as local and central balanced (MVHR) systems with various types of control systems. Sampling and data evaluation were handled by specialists. The monitoring part of the MONICAIR project is nearing completion and the first results are now available. The data show that (from the ventilation systems under investigation) systems with mechanical air supply and/or exhaust provisions for each individual room are more robust in maintaining certain IAQ-levels, whereas systems with only natural air exchange provisions in the habitable rooms show larger variations in IAQ and depend to a large degree on the number and behaviour of inhabitants. In dwellings with MEV-systems the CO2 concentration in the average habitable room is 380 ppm above the limit value of 1200 ppm during 10% of the heating season (515 of 5088 hours). If inhabitants spend only half of the time at home, then 20% of this time the air exchange is insufficient. Looking at individual houses these percentages vary from 2% to over 50%. In dwellings with MVHR systems the CO2 concentration in the average habitable room is 320 ppm above the limit value of 1200 ppm during 4.6% of the heating season (237 of 5088 hours). Again, if inhabitants spend half of the heating season indoors and at home, 9.2% of these hours have insufficient air exchange. Per dwelling this figure varies from 0.1% to more than 20%. Looking at the energy performance of the ventilation systems under investigation, monitoring data show that also in this respect significant differences occur. The average primary energy consumption for mechanical ventilation without heat recovery is 125 MJ/m2. For the ventilation systems with heat recovery the average primary energy consumption is 22 MJ/m2.