Directional solidification experiments on hypoperitectic Cu-Sn alloys have been performed at very low velocity in a high thermal gradient to ensure planar front growth of both phases. The diameter of the sample has been reduced to 500 mu m to strongly reduce convection. Lamellar and fibrous peritectic cooperative growth of the primary alpha- and peritectic beta-phases has been observed on length spanning several millimeters. For the first time in a high solidification interval peritectic alloy, a quenched interface of both phases in contact with the liquid has been obtained. An unexpectedly high volume fraction of the primary phase, which furthermore fluctuates over time, has been observed. This is attributed to the transient state of the (alpha + beta) growth front to a steady state and the associated evolution of the large diffusion layer ahead of the solid-liquid interface. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.