Thermogravimetry is used to investigate thermal degradation reactions of solids and to determine parameters of overall rate expressions. The validation of kinetic parameters for thermal decomposition of solids obtained from thermogravimetry by comparing the results with those from isothermal measurements is a necessity. This paper presents an isothermal gradient free reactor to measure thermal degradation of solids or liquids under isothermal conditions. Comparisons of literature and experimental data for polyamide 6 are made.