Quasicrystalline and crystalline phase transitions observed during in situ heating in the transmission electron microscope of amorphous thin films of the Al-Co-Cr-Fe-O system have been studied. In particular, heating the film at 750 degrees C during 2 h, grains with decagonal quasicrystalline structure were developed. This quasicrystalline phase presents a periodicity of 1.4 nm along the c-axis and its tenfold electron diffraction pattern indicates the existence of phason strains in its structure. X-ray energy dispersive spectroscopy microanalysis showed a high relative concentration of Al compared with the concentration of Cr, O and Fe in the quasicrystalline grains, and electron energy-loss spectroscopy confirmed the presence of oxygen in these grains. All this resulted in a nominal composition of Al58Cr28O17Fe5 for this phase (i.e. this is an oxygen-containing decagonal quasicrystalline phase). Cobalt was only detected in grains whose structure corresponds to crystalline phases such as Al13Co4, the well known approximant phase of the decagonal quasicrystal.