Abstract: Contours of H2O–content saturated to a mineral assemblage in the calculated p-T psaeudosections can provide much useful information on plausible evolution for the mineral assemblage. Guiraud et al. proposed that metamorphism in a closed system can only evolve via progressive dehydration, crossing contours of decreasing H2O–content, the fluid being lost from the rocks. If the metamorphic process crosses contours of increasing H2O–content, the mineral assemblage starts to become a fluid–absent state, which does not favor the further evelution of the mineral assemblage. Thus, a metamorphic peak should represent a point at which the metamorphic p-T path becomes tangential to a H2O–content contour or at which dehydration reactions complete and just start to rehydrate. It may not correspond to the maximum temperature and pressure reached along a p-T path. Using p-T pseudosections with contours of H2O–contents, the medium–low pressure metamorphism for pelite from the Altai orogen was documented. We proposed that the andalusite–type metamorphic zones in Altai were related to the uplifting of kyanite–type zones and are characteristic of isomorphic transformation between aluminosilicates. Generally, it is difficult for the low–pressure metamorphic assemblages to reach an equilibrium state. Distinctive from the low–medium pressure metamorphism of pelite, progressive dehydrations proceed even in the decompression process post metamorphic peak of UHP eclogites.