We investigated the temperature-dependent onset and subsequent reaction kinetics of spinel (MgAl2O4) interlayer growth in situ at T = 800-1000 degrees C in air by means of energy-dispersive as well as wavelength-dispersive synchrotron X-ray diffraction. We observed growth using a diffusion-reaction couple setup in which (111)-oriented periclase (MgO) single-crystal substrates reacted with initially amorphous Al2O3 thin films deposited via pulsed laser ablation. Microstructures and microtextures of the nanoscale reaction bands were analyzed ex situ using focused ion beam (FIB)-assisted transmission electron microscopy (TEM). Reaction bands grew topotactically into the substrates with the orientation relation (111) periclase parallel to (111) spinel and < 110 > periclase parallel to < 110 > spinel. We inferred temperature-dependent diffusion-controlled, mixed, and interface-controlled reaction kinetics from the increase of the integral intensity of the 111 spinel reflection during the in situ experiments. In case spinel formed, a porous layer at the periclase/spinel interface was found using TEM, displaying the negative reaction volume at this phase boundary. Results are compared with complementary experiments in which spinel growth was monitored using (0001)-oriented corundum (alpha-Al2O3, sapphire) substrates that reacted with MgO thin films [1]. The onset of spinel growth was observed at lower temperatures using periclase substrates, and an offset of about 100 K resulted in similar reaction kinetics. The positive reaction volume at the corundum/spinel interface was displayed by bend contours in TEM micrographs. Combined results suggest that the negative reaction volume at the periclase/spinel phase boundary has a crucial effect on the onset of spinel growth and subsequent reaction kinetics.