Seven gravity core samples were collected from the Tyrrhenian Sea in order to study the type of interstitial water as well as the diagenetic processes governing sedimentation in the western Mediterranean during the Holocene. This investigation revealed that no drastic changes in the chemical composition of interstitial water occurred in the carbonate sediments of the Tyrrhenian Sea which preserved their original type i.e. oceanic type (MgSO₄). Aerobic conditions prevailing during the Holocene may explain this phenomenon. Normal values of salinity (averaging 38) were found, except in the southern part of the sea where a value of 43.88 was recorded. The chemical composition of interstitial water of this region has been influenced, to some extent, by volcanic activity and by infiltration of brines from the underlying Messinian evaporites. Alkalinity was low for most of the investigated core samples. This can be attributed to the weak effect of biogenic processes such as oxidation of organic matter and sulphate reduction. The slight tendency for decreasing Alk, Ca^>2, Mg²⁺, Ca/Mg, Ca/Cl and Mg/Cl of interstitial water with depth in sediments for some core samples may reflect limited development of ion-exchange and adsorption processes between sediments and interstitial water.