The convective and diffusional mass transfer occurring in combined thermoanalyser-mass spectrometer systems can cause significant deviation between measured thermoanalytical and mass spectrometric curves. Based on experimental studies of the decomposition of CaCO3, a model has been developed which allows to interrelate the thermoanalytical (DTG) and mass spectrometric curves and provides a criterion defining under which conditions (carrier gas-flow rate and diffusivity of evolved gas) the disguising mass-transfer influences can be neglected. The criterion relates the total residence time tau(tot) of the gas in the experimental system to the characteristic time t(N) of the gravimetrically recorded decomposition process and allows to quantify the agreement between the thermoanalytical and mass spectrometric curves.