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The paper addresses the influence of the heat source capacity and structure on the accuracy of the thermophysical parameters measurements within several transient methods. First, the New Plane Source (NPS) method is improved by taking into account the heat source capacity. The results of measurement are compared with those of Step Wise Transient (SWT) method and the relative differences for glass and polymethylmethacrylate (PMMA) are better than 0.5 %. The second part is devoted to the Transient Plane Source (TPS) method. The results of measurement are compared with those of SWT method. The relative differences for glass are less than 0.5 %, but for PMMA 6 %, which is explained by specimen anisotropy. Finally, the reliability of the analytical model of TPS method is supported by comparing it with the numerical one obtained by Finite Elements Method (FEM), where three models of material composition of the heat source are used. The temperature functions are determined from numerical responses obtained by FEM and analytical solution. The coefficients of variation of all four determinations of the thermal diffusivity and conductivity of glass and PMMA are less than 0.4 %. [ABSTRACT FROM AUTHOR]