Microstructures and composition variations are described at quenched planar or cellular solid-liquid fronts in A1–2 wt% Cu. Steady state solute profiles obtained at different freezing rates from the quenched liquid are self consistent and correspond to a liquid diffusion coefficient of ∼ 3 × 10 -5 cm 2/ sec. Interfacial concentrations are also compatible with the relevant equilibrium diagram. Transient peaks of high copper content occur in the immediate vicinity of the quenched fronts and correspond to sharp transitions between planar and cellular and dendritic morphologies. The spacings of the initial products of quenching (cells or dendrites) seem to be determined during the quenching operation and do not change during subsequent growth. It is suggested that the initial spacing represents the wave length of successful perturbations of the steady state fronts, and that dendrite spacings do not subsequently change (a) because the growth velocity is not a sensitive function of this spacing, and (b) because crystallographic restrictions prevent gradual adjustment of dendrite spacings during growth. A possible relevance of these results to the structure of castings is noted.