In many countries, prescribed or planned burning is increasingly used as a management strategy to reduce the risk and negative effects of wildfires. As a by-product of this practice, ash, charcoal and partially charred material (referred to here as pyrogenic carbon, PC) is created. The amount and type of PC produced and fate of this form of carbon is uncertain. PC is often assumed to be resistant to chemical and microbial degradation and therefore potentially persistent in soils for hundreds or thousands of years. As a result, PC has been proposed as a sink for carbon and promoted for its storage potential in soil. We hypothesised that the differing components of PC would interact differently with soil processes and have varying potential for carbon storage. We analysed the chemical composition of PC produced by prescribed fire in a eucalypt forest and measured its effect on soil respiration. A laboratory incubation experiment showed that when PC of differing size fractions was added to soil, only the smallest size fraction (<1 mm; ash) increased rates of soil respiration, whereas larger fractions (charcoal) had little effect. The carbon contained in charcoal was resistant to microbial degradation and had little effect on microbial processes such as respiration. In general, fires of greater intensity will produce greater proportional amounts of smaller size particles and will likely result in faster rates of respiration than fires of lesser intensity. Therefore, lower intensity fires may ultimately have a greater capacity for soil carbon sequestration than those of higher intensity.