Summary The effect of heat treatment on rotational hysteresis ( WR- H) characteristics has been observed for samples of red sandstone, haematites and clay minerals. On heating the sandstones up to 930°C in air a steady rise in WR levels was observed. This process, which may be due to the production of cr-Fe203. pigment or an annealing effect (which is more probable), has an activation energy of about 0.2 eV/molecule. No magnetite was produced below 950 C but, after heating to 1200 C, a loss in cr-Fe,O,, pigment was observed together with a corresponding production of Fe30,. WR - H analysis of heated chamosite showed the production of magnetite at moderate temperatures (- 500 C). An increasing magnetic hardness ' was observed as specific surface increased for a series of pigment samples. This effect and the surprising observation that magnetic hardness increased after annealing may be consistent with the competing anisotropy ' model for haematite. The pigment and specularite content of the sandstones was estimated by comparison with W, - H curves for synthetic samples. Although these are not necessarily exactly like those occurring naturally, it is still possible to make comparative estimates for rock samples within a given formation. Although the number of samples in the present study is too few to draw positive conclusions, the results are not inconsistent with a correlation between W, - H and palaeomagnetic characteristics. The total amount of haematite (specularite plus pigment) is in moderately good agreement with values obtained by another method.