This International Standard specifies a method for the determination of the reactivity of unsaturated polyester resins as characterized by the maximum temperature reached by a solution of unsaturated polyester resin and catalyst when heated in a water-bath at 80 °C, by the time taken for the temperature of the mixture to rise from 65 °C to this maximum temperature, and also, if the maximum temperature exceeds 90 °C, by the time taken for the temperature to rise from 65 to 90 °C. These characteristics give an indication of the processing behaviour of unsaturated polyester resins and can be used to compare resins of similar type. As the test results depend very much on the conditions under which the test is carried out, these conditions are described in detail NOTES 1 If the method is used to test commercial benzoyl peroxide catalyst, the results should be compared with those obtained for the same resin using pure benzoyl peroxide. 2 By agreement between the interested parties, certain test conditions can be modified : such as the test temperature (temperature of the thermostatically controlled bath) and the quantity and type of catalyst system (mixtures of catalysts, combinations of catalysts and accelerators).