Observations by transmission electron microscopy (TEM) of the submicrometer phases present in calcium aluminate cements have shown that Ca-Al-Fe oxides coexist in two forms with brownmillerite (b) and perovskite (p) structures, respectively. Homogeneous single crystals of both brownmillerite and perovskite have been observed but exsolved lamellae also occur on the scale of tens of nanometers. Perovskite lamellae in brownmillerite exhibit coherent interfaces with an almost perfect [1 0 1](b) = {1 0 0}(p) topotactic relationship Energy-dispersive X-ray spectroscopy (EDXS) measurements show that perovskite lamellae are enriched in Ti and Si relative to the brownmillerite lamellae. The perovskite phase may accommodate up to 0.17 Si atoms per formula unit, but the exsolution process seems mainly to concern the Ti content. It is estimated that the solvus width ranges between concentrations of 0.06 < Ti < 0.13 atoms per formula unit. O K and Fe L-2,L-3 edges collected by electron energy loss spectrometry (EELS) confirm that both phases are mainly composed of Fe3+ requiring that the perovskite is highly oxygen-deficient. Al K and Si K EELS spectra have features comparable with those of fourfold-co-ordinated Al and Si sites. suggesting that they are probably located close to oxygen-vacant sites.