This article describes the character of thermal decomposition of peat and peat humic acids of two peat profiles from ombrotrophic bogs in Latvia in relation to their origin. The results of differential thermal analysis show that the character of peat decomposition significantly depends on the composition, age, and origin of peat. Studies of the parameters of thermal decomposition help to identify the differences between peat and the corresponding humic acids, as well as the links between the peat age, decomposition degree, and character of decomposition process, thus ascertaining maturity of organic material. Accordingly, thermal analysis methods can be used for identification of the origin of peat and peat humic acids. Thermal treatment of peat helps to accelerate the changes in the peat properties and thus significantly increase the oil sorption capacity. Thermally treated peat is much more hydrophobic and could be considered as a prospective sorbent for oil sorption.