A comparison of the high field magnetic properties of small magnetic spheroids associated with some marine Cretaceous/Tertiary (K/T) boundary clays and of spheroidal or glassy byproducts of impact indicates significant differences. Tektite glass may have a similar room-temperature coercivity spectrum, but low-temperature measurements reveal the presence of abundant very fine superparamagnetic grains. Ablation spherules are significantly more magnetic than K/T spherules, and their isothermal remanence acquisition and demagnetization behaviors also differ from the “single domain” character of the boundary spherules. Impact glass from terrestrial craters and upper Eocene clinopyroxene spherules exhibit significantly higher remanent coercive force values, probably reflecting compositional differences. Among nonimpact materials, industrial fly ash represents a close magnetic analogue to the K/T boundary magnetic forms. The K/T spherules also share some morphological and chemical features with magnetic spherules present in fly ash. Magnesioferrite and Fe-spinel, two rare natural minerals present in the K/T spheroids, have been associated with naturally combusted bituminous shales. This suggests that a hypothesized global combustion event synchronous with the termination of the Cretaceous may have also, in part, involved the combustion of fossil fuel.