Despite the influence that paleomagnetism has exerted on the earth sciences, our understanding of thermal remanent magnetization (TRM), which is one of the primary sources of paleomagnetic information, is at best fragmentary and incomplete. In this paper we report on experiments studying TRM properties of magnetites whose particle sizes vary from single domain (SD) to pseudo single domain (PSD) to multidomain (MD). TRM stability has been measured by using alternating field (af) demagnetization, hot af demagnetization, low-temperature treatments, thermal demagnetization, and storage tests. Low-temperature treatments are a great aid in helping to determine whether a sample's TRM is carried primarily by SD, PSD, or MD magnetite particles. Thermal demagnetization is relatively insensitive for predicting particle sizes, while af demagnetization remains the best method for reducing the MD contribution to the remanence. Theoretical considerations indicate that the thickness of a domain wall should initially increase with temperature until a critical value is reached, beyond which the thickness decreases. This result is useful in the interpretation of the hot af demagnetization experiments of MD particles whose median destructive alternating field increases with temperature up to some maximum value before decreasing, while the median destructive field of SD and PSD particles decreases monotonically with temperature. Characteristics of the Lowrie-Fuller test (Lowrie and Fuller, 1971) for equidimensional 0.2-μm-sized particles indicate that a critical TRM-inducing field hc exists where 0.10<hc<0.49 Oc; SD-like behavior is exhibited when the inducing field is greater than hc, and MD behavior is exhibited for fields less than hc. This suggests that the critical size for transitions of domain structure depend on the intensity of the inducing field. It is argued that the presence of an external field and, similarly, blocking at high temperatures increase the effective size region of SD/PSD remanence and that remanence might be blocked in grains with a nonequilibrium domain configuration. Estimates of critical sizes for transitions between SD, PSD, and MD behavior depend on several factors including grain shape, temperature, type of remanence given to the sample, mineralogy, and intensity of the field used to induce the remanence.