Defines a procedure for determining the reactivity of unsaturated polyester resins characterized by the maximum temperature reached by a solution of unsaturated polyester resin and catalyst when heated in a water bath at 80 deg C, by the time taken for the temperature of the mixture to rise from 65 deg C to this maximum temperature; in addition, if the temperature is in excess of 90 deg C, by the time taken for the temperature to rise from 65 - 90 deg C. Can be used for comparison of resins of similar type.