The thermal decomposition of anhydrous dicarboxylic acids was studied systematically in atmospheres of Ar, CO2 and air by means of thermogravimetry (TG) and differential thermal analysis (DTA). For all the acids, it was shown that CO2 has an inhibiting effect on the decomposition, whereas air has an accelerating effect, with respect to the behaviour in Ar. Except for oxalic acid, plots of the initial decomposition temperatures T(i) and the DTA peak temperatures T(m) versus the number of CH2 groups in the acids have a saw-tooth or periodic pattern. It is assumed that their properties depend on the zig-zag structures of the dicarboxylic acids.