The impact of increasing atmospheric CO2 concentration and associated warming on soil carbon (C) remains an open topic for debate. Here we present a study of the temperature response of soil organic matter decomposition (RS) using soil from a grazed grassland exposed to 23 years of experimental CO2 enrichment.  Macro-molecular rate theory (MMRT) was used to describe RS in incubations of soil at temperatures from 4 – 48° C.  No differences in either rate or temperature response of RS were observed due to the CO2 treatment. Below 40° C, characteristics of the temperature response indicated physiochemical rather than enzymatic constraints to the increase in RS with temperature.  An optimum temperature (Topt) for RS was observed between 44 – 46° C. However, this was not captured by MMRT, which exhibited a Topt at 107° C, suggesting a biological limit to RS not represented in MMRT.  Our results support the role of substrate supply in driving the rate and temperature response of RS.  These results apparently contrast with those from the first 10 years of the CO2 treatment, which showed increases in soil C stocks, organic C concentration, labile C, root inputs and RS under elevated CO2.  Further research is needed to identify drivers of the long-term response of RS and soil C to elevated CO2 and temperature.  This information will support adaptation of grazing management strategies to maintain or increase soil C stocks under future climate scenarios.