Cross-spectral techniques are used to analyze the relationship between gravity and bathymetry at the Mid-Atlantic Ridge and the East Pacific Rise crests. The resulting transfer functions were used to study the nature of the isostatic mechanism operative at these ridge crests. The most satisfactory results were obtained for models in which the oceanic lithosphere is treated as a thin elastic plate overlying a weak fluid. The best fitting elastic thickness to explain gravity and bathymetry at the fast spreading (&ngr;>5 cm/yr) East Pacific Rise is in the range of 2--6 km and at the relatively slow spreading (&ngr;80 m.y.) parts of the oceanic lithosphere. This difference is consistent with the fact that ridge topography is formed near the ridge axis, where isltherms are shallower and the lithosphere is thus weaker than in older regions. The difference between the elastic thickness of the East Pacific Rise and Mid-Atlantic Ridge is significant and may represent differing temperature structures at these ridges. Simple models in which it is assumed that the elastic thickness represents the depth to the 450¿C isotherm show that these variations can be explained by differences in the spreading rate at these ridges. Thus the lower effective thickness at the East Pacific Rise can be attributed to higher average temperatures at shallow depths in a region surrounding the ridge crest. This is due to the faster spreading rate which results in isotherms having a shallower dip away from the axis than at the slower spreading Mid-Atlantic Ridge. This model cannot, however, explain gravity and bathymetry data over the Rekyjanes Ridge. The best fitting elastic thickness for this slow spreading ridge is similar to the thickness determined for the East Pacific Rise, suggesting an anomalous thermal regime at this ridge crest.