Simultaneous temperature-programmed desorption, mass spectroscopy and thermogravimetry (TPD–MS–TG) was applied to the well-defined spherical hematite particles, with various sizes and degree of aggregation of primary polynuclear (PN) subunit particles, produced by the forced hydrolysis reaction. TPD–MS–TG revealed that all the hematite particles produced from the forced hydrolysis reaction possess not only adsorbed H₂O and CO₂ and surface OH groups but also OH⁻ ions within the particles. This confirmation of the existence of lattice OH⁻ ions provides some validation that the hematite particles produced by the forced hydrolysis reaction were formed by the aggregation of PN particles. The elimination temperature of adsorbed H₂O and CO₂, surface OH groups and lattice OH⁻ ions depended strongly on the micropore structure and degree of aggregation of the PNs. The release of Cl was also observed. It is suggested that the structure of micropores and the degree of aggregation of the subunits in colloidal particles could be estimated by this TPD–MS–TG method.