We describe three useful techniques for investigating magnetic microstructure and granulometry, high-temperature alternating-field (AF) demagnetization, thermal fluctuation analysis (TFA), and determination of the demagnetizing factor N, and give examples of their use. For synthetic magnetite samples, thermoremanent magnetization and anhysteretic remanent magnetization are shown to differ in their AF demagnetization spectra at all temperatures up to the blocking range. Moreover, various coercivity fractions of either spectrum evolve differently with temperature than does the bulk coercive force. TFA of high-temperature coercive force data for dispersed magnetite crystals with mean sizes of 215, 390 and 540 nm indicate that ≈200 nm regions are thermally activated in each case. These regions may be responsible for pseudo-single-domain behaviour. A crude estimate of N for multidomain grains at any temperature is the ratio between bulk coercive force and saturation remannence. The 215--540 nm magnetites appear to contain two or three domains between -100¿ C and 400¿ C; new domains may nucleate at higher temperatures. ¿ American Geophysical Union 1990