Abstract: Muscovite is one of the most widespread minerals in metamorphic rocks. In most mineral assemblages, the Si content in phengite increases with increasing pressure, which can be used as a good geobarometer. Experimental study of the relations between the Si content of phengite and p-T conditions in three KMASH linfited assemblages indicates that the geobarometer of the Si content of phengite is highly dependent on mineral assemblages. Consequently, we cannot simply extrapolate the experimental results to determine the metamorphic pressures of natural rocks with assemblages different from the experimental ones. Therrnopdynamic simulation of the phengite Si isopleths in the KNIASH p-T pseudosections shows a good a- greement with the experimental results of high quality. Hence, we can use this approach in the KFMASH or more comphcated systems. It is preferentially recommended to use the pT pseudosection approach contoured with phengite Si isopleths to define metamorphic pressures of natural mineral assemblages.