The exothermic chemical conversion of exhaust gases leads to a spatially and temporarily varying heat source in the monolithic structure of three-way catalytic converters which results in characteristic radial and axial temperature profiles. In an engine-test bench, temperature profiles are measured and compared for fresh and aged catalysts, the latter ones aged by real exhaust gases at steady-state and transient real conditions as well as by a hot inert gas. Numerical simulations of the heat and mass transport and chemical conversion are used to reproduce the measured temperature profiles assuming locally varying dispersion of the catalytic components due to aging without knowing the aging history. Hence, the local chemical heat source, observed by the measurement of temperature profiles, can directly be used to estimate the local catalyst deactivation, expressed by the reduction of the local catalyst dispersion.