The present indoor environmental control is implemented according to indoor environmental parameter set points. However room occupants generally lack of the knowledge on comfortable values of indoor environmental parameters. The next generation of indoor environmental control should be based on room occupants’ actual perception instead of set points. This paper describes some fundamental work for developing such a system. Experiments were conducted to collect room occupants’ complaints when they feel dissatisfied to the indoor environment. The collected data were analyzed to find the characteristics complaining behaviors.