Sea surface magnetic and bathymetric data from two areas of observed high-temperature hydrothermal activity in the Atlantic (trans-Atlantic geotraverse, or TAG hydrothermal field) and Pacific (Sea Cliff hydrothermal field) and two areas of inferred hydrothermal activity in the Atlantic (Mid-Atlantic Ridge (MAR) at 15°N and 17°N) were studied to determine the effect of vent activity on magnetization of the oceanic crust. All of the sites examined show a low in the magnetic anomaly in varying degrees. Magnetization lows within the axial marine magnetic anomaly have been associated with some areas of active and relict high-temperature hydrothermal venting. A three-dimensional magnetic inversion was performed on magnetic and bathymetric data from the TAG hydrothermal area on the Mid-Atlantic Ridge (26°N), and two-dimensional magnetic inversions were done on individual profiles from the other study sites. The map from the TAG area shows a “negative” in the crustal magnetization solution just to the east of the median valley axis which encompasses the active hydrothermal area. A low in magnetization continues up the eastern wall which suggests alteration of the crust. A distinct low in magnetization is present within the axial anomaly at the Sea Cliff hydrothermal field located on the eastern wall of the Gorda Ridge. The magnetization low becomes less pronounced farther south of the known vent field. Profiles from 15°N and 17°N on the Mid-Atlantic Ridge show magnetization lows over areas of inferred hydrothermal activity. These lows are present over the the eastern and western walls of the median valley, respectively. The lows observed in the axial marine magnetic anomaly in these four study sites all occur slightly off axis, on crust that is at least 200,000 years old on the Mid-Atlantic Ridge and at least 100,000 years old on the Gorda Ridge. On the basis of magnetic modeling and measurements of rock magnetic properties, the most probable cause of the low in the magnetization is alteration of the oceanic crust with other subsidiary contributing factors which vary from site to site. The magnetization low over TAG is most probably due to high-temperature hydrothermal alteration of the crust and a thinner layer 2, with possible contributions from reversal events and magma chamber/Curie isotherm effects. The low near the Sea Cliff hydrothermal field on the Gorda Ridge is probably the product of tectonically controlled low-temperature (nonhydrothermal) alteration, high-temperature hydrothermal alteration and magnetic reversal events. At 15°N and 17°N on the MAR, the lows may be produced by alteration of the crust due to possible hydrothermal activity.