It is shown that the thermal decrease of the spontaneous magnetization ofiron differs characteristically depending on the preparation conditions.In all cases, the deviations from saturation at absolute zero can beaccurately described by a single temperature power term Tε whichholds up to several hundreds of kelvins. The empirical exponents are ε = 2 for crystalline bulkiron, ε = 3/2for amorphous iron as well as for isotropic (crystalline) two-dimensional iron films, butε = 5/2for inhomogeneous (pseudomorphic) monatomic layers. In crystalline films which aresufficiently thick (∼100–300 nm) to allow the excitation of standing spin waves, theT2bulk law is confirmed for the uniform precession mode, but for states with aperiodically modulated magnetization due to spin wave resonance, a T3/2dependence holds. For ultrathin iron films with more thanthree atomic layers, a thermodynamic crossover from T3/2 toT2 isobserved at higher temperatures. This is indicative of a dimensionality change of therelevant interactions from 2 to 3. Crystalline and amorphous bulk iron also exhibit acrossover, but only for the amplitude and not for the exponent of the Tε law.This is assumed to be caused by the thermal variation of the magneticinteraction strength. All observed empirical exponents ε fitinto a recently proposed scheme of universality classes for the low-temperaturebehaviour of the order parameter in materials with half-integer spin quantum numberand three-, two-and one-dimensional interactions. It can therefore be concludedthat the thermodynamics of iron is like that of an insulating material withs = 1/2.