As the first part of a series on kinetic studies on the thermal decomposition of iron(III) hydroxide oxide, alpha-FeO(OH), involved in a crustal material, banded iron formations, is subjected to thermogravimetric measurements at various constant and linearly increasing temperatures. The validity of the kinetic procedures employed in a series of kinetic studies is tested by a comparative kinetic study of the thermal decomposition of alpha-FeO(OH) involved in two characteristic parts of banded iron formations, in which 54.9 +/- 1.2 and 44.5 +/- 2.3 wt% of alpha-FeO(OH) are involved. Under isothermal conditions, kinetic agreement with an Avrami-Erofe’ev equation A(m), (m approximate to 1.5) was observed for the thermal decomposition of both samples to alpha-Fe2O3. The Arrhenius parameters calculated for the two samples are identical within the statistical error. For linearly increasing temperatures, the apparent activation energies calculated for these samples by a different isoconversion method were different by about 35 kJ mol(-1). Kinetic agreement with A(1.5) for the sample containing 54.9 wt% of alpha-FeO(OH) was identical to that found under isothermic conditions. The kinetics of the other sample were characterized by the larger apparent Arrhenius parameters and the kinetic agreement with the A(1) law. The different kinetic behavior under non-isothermal conditions of the samples was probably due to the difference in diffusional removal of evolved water vapor, depending on the compositional difference in the two characteristic parts of banded iron formations.