This paper presents an experimental investigation into the effect of different factors, such as contaminants, humidity content, temperature, field strength and frequency on water treeing in polyethylene (PE) and cross linked polyethylene (XLPE). For this investigation, model test specimens have been used to supplement cables for testing. Colored trees and their points of initiation have been analyzed for their content of impurity elements and always are found to contain sulfur and some metal. The results show that the relative humidity in the insulation must exceed approximately 70% to produce a significant number of water trees. The average size of the trees shows a very small dependence upon time and temperature and a slight decrease with increasing field strength. An increase in frequency from 50 to 5000 Hz approximately doubles the average size of the trees. The trees appear during the first 300 hours of voltage application, but since no growth rate can be observed at either frequency, it is not justified to establish a frequency acceleration factor in general.