The objective of this paper is to present reference experimental measurements of the morphological stability threshold velocity and cellular growth undercooling for two alloy compositions in the tin bismuth (Sn-Bi) metallic system. The measure is based on an original in situ diagnostic that relies on the Seebeck thermoelectric effect and benefits from the microgravity environment to guarantee diffusive solute transport conditions. When analysed using independently determined thermo-physical parameters, the experimental data is found to support the validity of the Mullins and Sekerka theory. Above the morphological stability threshold, the data can be interpreted assuming that the cell tips progress along the temperature gradient until the local driving force for morphological instability becomes negligible. (c) 2005 Elsevier B.V. All rights reserved.