The aim of presented research is to identify factors that determine 8 wages in Poland and to find out if gender pay gap exists, applying 9 classification trees. For the grouping variable i.e. net income from the main 10 place of employment we construct clusters of respondents that are created 11 due to such features as: gender, education, employment contract, economic, 12 occupation, additional job, size of enterprise, measured by the number 13 of employees, age and job seniority in years. Investigation is provided 14 applying data from the Polish Labor Force Survey in the years 2003, 2006 15 and 2009.