Interface pattern evolution during directional solidification has been investigated in the microgravity environment of a space shuttle mission providing diffusive conditions for cellular pattern evolution. A model transparent alloy of succinonitrile-acetone was used in these studies for in situ observation of the dynamic process of pattern evolution. In addition to the microgravity environment, alloy composition and growth condition experiment parameters were selected in such a way that the planar-to-cellular transition was supercritical, and pattern evolution was examined dose to the transition velocity that gave rise to very small amplitude cells. Since the supercritical transition shows continuous increase in amplitude with velocity, non-linear effects in wavelength selection are small. This means that linear theory could be used to interpret the results. The evolution of primary spacing and the order of cellular arrangements were characterized as a function of time for three different velocities. No sharp wavelength selection in primary spacing was observed in the experimental results, and the distribution of steady-state spacing was characterized for each velocity. Wavelength selection for supercritical transition near the transition point can also be explained by stability analysis.