Warming in the boreal forest region will increase fire size, severity, and frequency. This may result in peatlands making up a larger proportion of future burned area, shifting them from a net sink of atmospheric carbon (C) to a net source. In addition, mercury (Hg) that accumulates in peat is susceptible to re-release to the atmosphere during peat fires. Here I investigated the physical properties driving depth of burn and emissions of C and Hg. I found the interaction between bulk density and water content to be the most important factor in predicting depth of burn, CO2, and CO emissions. This was not the case for CH4 and Hg emissions. These results also indicate that previously reported CO:CO2 and CH4:CO2 emission ratios are underestimating C emissions from smouldering peat. The quantity of C released from burning peat underscores the importance of considering fire in peatland C budgets.