Coexistence of multiple lamellar thicknesses and lamellar thickness changes in poly(ether ether ketone) (PEEK) have been supported by the results obtained from investigations using differential scanning calorimetry (d.s.c.), X-ray and scanning electron microscopy. The melting behaviour, crystallization kinetics and morphology of PEEK subjected to multiple-step crystallization have been studied. During stepwise crystallization of the polymer, a major lamella crystal develops first, which possesses a melting peak temperature close to the equilibrium melting temperature; however, a small fraction of the polymer is packed into minor crystal aggregates of smaller lamellar thicknesses which, upon d.s.c, display the observed phenomenon of multiple minor melting peaks located further below the main melting peak. It is this multiple distribution of lamellar thicknesses that causes multiple melting behaviour. A descriptive crystallization kinetic model is used to help explain this phenomenon by correlating the crystallization and thermal behaviour in PEEK.