A possible cause of the wide differences present in the literature values for the calorimetric measurements of the pyrolysis heat of biomass was analyzed. The apparent specific heat of char from pyrolysis of beech wood was investigated by means of a differential scanning calorimeter (DSC). The objective was to find out the influence of the use of a lid on the crucible on the experimental results. In a first experimental series, the crucible containing the sample and the empty reference crucible were closed by a pierced lid. In a second experimental series, the crucible containing the sample and the reference crucible were used without a lid. Results of experiments within the same series showed excellent agreement, but the results of the first series were completely different with respect to those of the second series. A possible explanation for these differences are the different emissivities of the crucible material (aluminium) and char. During experiments without a lid a different radiative heat exchange occurs between the two crucibles and the opposite wall of the DSC measurement cell. In order to confirm the effect of heat radiation inside the DSC measuring cell, additional experiments with aluminium oxide and graphite were carried out.