This document specifies a method for determining the glass transition temperature, Tg, 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 Tg is determined from the peak in the tan δ versus temperature curve. Glass transition temperature, Tg, determined in this way serves the purpose of a guideline to the service temperature of the material.