Due to the increasing environmental concerns, the use of fossil fuels is being reduced. In search of less polluting ways of energy production, co-combustion offers a good way to decrease emissions and decrease the use of fossil fuels. As very high temperatures and heating rates are used in the industry, this paper presents the preliminary results of the co-combustion of the different ratios of mixtures of wood and oil shale at very high heating rates (up to 1000 °C min−1). Both oil shale and wood were analysed separately, too. The results showed that in case of wood, a higher heating rate increases the temperature range of the second decomposition step, also complicating its distinction. For the mixtures, ignition was shifted to significantly lower temperatures when comparing to oil shale. The end temperatures remained less affected.