The constituents of contamination on insulators surface under field condition are affected by various factors such as terrain

climate

and insulator surface material. In order to investigate the pollution constituents on the surface of external insulation

we studied a method based on ion exchange chromatography to measure the ionic composition of natural contamination on ceramic

glass

and composite insulators in Chongqing area. Aiming at the weakness of ion-pairing

we put forward a rule of ion pairs based on fuzzy cluster analysis

providing an objective basis for measuring ion-pairing of pollution constituents. The results show that CaSO

4

is the primary constituent on insulators in Chongqing. The characters of material can lead to difference between pollution c

onstituents and contents on insulators with different surface material. equivalent salt deposit density（ESDD） cannot characterize the pollution level of external insulation in field condition. A set of characterization method basing on pollution constituents needs to be put forward to enhance the reliability of the contaminants measurement in power system external insulation.