The micromechanical properties (microindentation hardness, H, elastic modulus, E) of poly(ethylene terephthalate) (PET), isothermally crystallized at various temperatures (T-a) from the glassy state are determined to establish correlations with thermal properties and nanostructure. Analysis of melting temperature and crystal thickness derived from the interface distribution function analysis of SAXS data reveals that for T-a < 190 degrees C the occurrence of two lamellar stack populations prevails whereas for samples annealed at T-a < 190 degrees C a population of lamellar stacks with a unimodal thickness distribution emerges. The H and E-values exhibit a tendency to increase with the degree of crystallinity. The results support a correlation E/H-20 in accordance with other previously reported data. The changes of microhardness with annealing temperature are discussed in terms of the crystallinity and crystalline lamellar thickness variation. Unusually high hardness values obtained for PET samples crystallized at T-a = 190 degrees C are discussed in terms of the role of the rigid amorphous phase which offers for the hardness of amorphous layers constrained between lamellar stacks a value of H-a similar to 150 MPa. On the other hand, for T. 240 degrees C the decreasing H-tendency could be connected with the chemical degradation of the material at high temperature. (c) 2005 Elsevier Ltd. All rights reserved.