This document specifies a method for determining the glass transition temperature, T_g, of vulcanized rubbers in the hardness range from 30 IRHD to 80 IRHD. The dynamic properties are measured via temperature sweep in sinusoidal deformation at a defined strain and frequency and T_g is determined from the peak in the tan δ versus temperature curve. Glass transition temperature, T_g, determined in this way serves the purpose of a guideline to the service temperature of the material.