Planar [00. l] disorder in sodium beta alumina

00./and 01./ lattice images are used to show the presence of OO.l] disorder in alumina. It is shown that both stacking sequence faults and intergrowth are present. The faults are believed to be introduced in the early stages of sintering when spinel block ledges are added out of sequence during growth in the 00. l] direction. The observations show that the planar disorder observed by Bevan et al. 4] cannot be attributed solely to heterophase intergrowth.