Rod eutectic growth in succinonitrile-(D)camphor is investigated experimentally using directional solidification with specimen thickness ranging from 20 to 200 mu m. Measurements in the projected plane are reconciled with a model for array distortion under the constraint imparted by the geometry, and the influence of geometry is examined with respect to the two dimensional (2D) array basis vectors. A transition from 3D to quasi-3D growth at very low velocity is observed, where specimen thickness becomes comparable to eutectic spacing. The principal finding is a velocity-dependent but thickness-independent transition in array orientation in the "bulk" regime. The transition is correlated with the dominant Unstable mode in the initial succinonitrile front, and we assert that this instability controls the array spacing along the slide wall direction. It is concluded that this type of analysis is required tor meaningful interpretation of experimental results regarding rod eutectic growth. even in the apparent thick-slab or "bulk" regime. (c) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.