Alteration of ocean crust is a fundamental on-going geological process. High-temperature alteration of the ocean crust occurs at the ridge, while diffusive, low temperature alteration is present off-ridge. Both alteration processes produce secondary minerals and change the primary chemical and isotopic signal of the oceanic crust. These signals can be brought into the mantle as altered oceanic crust enters a subduction zone and produces mantle inhomogeneities. As molybdenum is low in abundance in the mantle, but high in abundance in seawater with a distinct isotopic signal from each other, it is a promising element to study these processes. Prior to study deep cycling of molybdenum via ocean island basalts, detailed understanding of the behavior of molybdenum during alteration is needed. In this framework, we propose a study to identify the major molybdenum-bearing phase as well as the processes influencing the molybdenum isotope signal of the altered oceanic crust. Two cross-sections through the oceanic crust will be studied for this purpose, ODP Site 1256 and Macquarie Island (14 Ma and 7 Ma old, respectively). In-situ molybdenum concentrations and whole rock molybdenum isotope measurements are planned to understand on- and off-ridge alteration in the oceanic plate. This research will be groundbreaking, using molybdenum as a novel tracer to better constrain the behavior of on-/off-ridge alterations and to quantify the Mo-isotopic composition of the oceanic crust potentially entering the subduction zone.