The Southeast Indian Ridge exhibits a transition in axial morphology from an East Pacific Rise-like axial high near 100 ¿E to a Mid-Atlantic Ridge-like rift valley near 116 ¿E but spreads at a nearly constant rate of 74--76 mm/yr. Assuming that the source of this transition lies in variations in mantle temperature, we use shipboard gravity-derived crustal thickness and ridge flank depth to estimate the variations in temperature associated with the changes in morphological style. Within the transitional region, SeaBeam 2000 bathymetry shows scattered instances of highs, valleys, and split volcanic ridges at the axis. A comparison of axial morphology to abyssal hill shapes and symmetry properties suggests that this unorganized distribution is due to the ridge axis episodically alternating between an axial valley and a volcanic ridge. Axial morphology can then be divided into three classes, with distinct geographic borders: axial highs and rifted highs are observed west of a transform fault at 102¿45' E; rift valleys are observed east of a transform fault at 114 ¿E; and an intermediate-style morphology which alternates between a volcanic ridge and a shallow axial valley is observed between the two. One segment, between 107¿ and 108¿30' E, forms an exception to the geographical boundaries. Gravity-derived crustal thickness and flank depth generally vary monotonically over the region, with the exception of the segment between 107 ¿E and 108¿30' E. The long-wavelength variations in these properties correlate with the above morphological classification. Gravity-derived crustal thickness varies on average ~2 km between the axial high and rift valley regions. The application of previous models relating crustal thickness and mantle temperature places the corresponding temperature variation at 25 ¿C--50 ¿C, depending on the model used. The average depth of ridge flanks varies by ~550 m over the study area. For a variation of 25¿--50 ¿C, thermal models of the mantle predict depth variations of 75--150 m. These values are consistent with observations when the combined contributions of crustal thickness and mantle density to ridge flank depth are considered, assuming Airy isostasy. Crustal thickness variations differ at the two transitions described above: A difference of 750 m in crustal thickness is observed at the rift valley/intermediate-style transition, suggesting small variations in crustal thickness and mantle temperature drive this transition. At the axial high-rifted high/intermediate-style transition, crustal thickness variations are not resolvable, suggesting that this transition is controlled by threshold values of crustal thickness and mantle temperature, and is perhaps related to the presence of a steady state magma chamber. ¿ 1998 American Geophysical Union