Experimental investigations of the atomic structure of topotaxial solid-solid reaction fronts were performed. XRD, TEM, SAED, EDS, RBS and specifically cross-sectional high-resolution electron microscopy (HRTEM) were applied to solid-solid reaction interfaces of a number of topotaxial model systems. The reaction kinetics of a silicide-forming reaction was shown to depend on the orientation of the silicide NiSi2. HRTEM revealed a corresponding difference in the atomic structure of the reaction front. The reaction mechanism of spinel-forming reactions was found to be determined by the spinel/oxide lattice misfit which at the moving reaction front has to be steadily accommodated. Sign and degree of the lattice misfit at the reaction front were systematically varied in the experiments by studying a series of different spinels, viz. Mg2TiO4, MgFe2O4, MgAl2O4, MgCr2O4 and MgIn2O4, revealing interfacial dislocations of different types. Respective results are discussed in terms of a model featuring the interplay between kinetic reaction barriers, the atomic mechanism of the solid state reaction and the properties of the interfacial defects (ledges, dislocations) persisting at the moving reaction front.