The microstructure of two commercial pipeline steels X52 and X65 was examined to provide a foundation for the understanding of the IGSCC mechanism of pipeline steels. Observation of the microstructure was carried out using scanning electron microscopy (SEM) and an analytical electron microscope. The microstructure of X52 and X65 pipeline steels shows banding of pearlite rich and ferrite rich areas. The ferrite grains were about 10 mu m in size with curved grain boundaries. There was carbide at the ferrite grain boundaries for X52 steel, and there was circumstantial evidence to suggest carbon segregation at the boundaries. The pearlite colonies were consistent with nucleation by a number of different mechanisms.