Results are presented for the first in-depth investigation of TI isotope variations in marine materials. The TI isotopic measurements were conducted by multiple collector-inductively coupled plasma mass spectrometry for a comprehensive suite of hydrogenetic ferromanganese crusts, diagenetic Fe-Mn nodules, hydrothermal manganese deposits and seawater samples. The natural variability of TI isotope compositions in these samples exceeds the analytical reproducibility (?? 0.05???) by more than a factor of 40. Hydrogenetic Fe-Mn crusts have ??205TI of + 10 to + 14, whereas seawater is characterized by values as low as -8 (??205TI represents the deviation of the 205TI/203TI ratio of a sample from the NIST SRM 997 TI isotope standard in parts per 104). This ~ 2??? difference in isotope composition is thought to result from the isotope fractionation that accompanies the adsorption of TI onto ferromanganese particles. An equilibrium fractionation factor of ?? ~ 1.0021 is calculated for this process. Ferromanganese nodules and hydrothermal manganese deposits have variable TI isotope compositions that range between the values obtained for seawater and hydrogenetic Fe-Mn crusts. The variability in ??205TI in diagenetic nodules appears to be caused by the adsorption of TI from pore fluids, which act as a closed-system reservoir with a TI isotope composition that is inferred to be similar to seawater. Nodules with ??205TI values similar to seawater are found if the scavenging of TI is nearly quantitative. Hydrothermal manganese deposits display a positive correlation between ??205TI and Mn/Fe. This trend is thought to be due to the derivation of TI from distinct hydrothermal sources. Deposits with low Mn/Fe ratios and low ??205TI are produced by the adsorption of TI from fluids that are sampled close to hydrothermal sources. Such fluids have low Mn/Fe ratios and relatively high temperatures, such that only minor isotope fractionation occurs during adsorption. Hydrothermal manganese deposits with high Mn/Fe and high ??205Ti are generated by scavenging of TI from colder, more distal hydrothermal fluids. Under such conditions, adsorption is associated with significant isotope fractionation, and this produces deposits with higher ??205TI values coupled with high Mn/Fe. ?? 2002 Elsevier Science B.V. All rights reserved.