Fuzzy set theory can be used to study the relationship between the glass-transition temperature (T-g) and structure of polymers. We used the method to map this relationship and obtained T-g's for 241 polymers with a standard deviation of 20 K (the confidence bound was 90%). We also used the method to predict T-g's for 15 polymers with a standard deviation of 67 K (the confidence bound was 90%). This study demonstrates that fuzzy set theory can be effectively used for determining the quantitative structure-property relationship of polymers.