Great interest surrounds the use of biochar as an agent for long-term soil carbon (C) sequestration due to its perceived high stability in a terrestrial environment. However, there is still little known about factors affecting its decomposition in soil and even less is known about the effect of labile organic matter (LOM) on biochar decomposition and the effect of biochar on the mineralization of LOM. To quantify the short-term interactive priming effects of LOM on carbon mineralisation in a smectite rich soil (Vertisol), we conducted a 4-month long incubation study. Sugarcane mulch (applied at 0, 1, 2 and 4 % (w/w)) was combined with biochar produced from 13C-depleted Eucalyptus salinga wood pyrolysed at either 450 oC or 550 oC. The use of isotopically-distinct biochar-C (δ13C = ~ -36‰) and LOM (-12.7‰) and/or soil-C (-14.3‰) allowed the separation of sources (biochar-C and LOM/soil-C) of CO2 evolution during incubation. The addition of LOM increased the initial decomposition rate of the small labile fraction of biochar-C. The cumulative biochar-C mineralised was greater in the 4% LOM treatment compared to the other treatments; however the total biochar-C mineralised over 4 months was relatively small, with values ranging between 0.4 and 1.0% of the added biochar-C across the treatments. Biochar produced at the lower temperature mineralised to a greater extent than the higher temperature biochar at all application rates of LOM. The presence of biochar had a negative priming effect on the LOM i.e. biochar decreased the decomposition of LOM in the soil and the effect became more obvious at the high addition levels of LOM. Biochar appears to be quite stable in soil in the presence of LOM and biochar application seems to be stabilizing the added LOM in the soil, however, long-term field studies are required to confirm these results.