The onset of cellular solidification in a thin-film solidification system using dilute acetone in succinonitrile is experimentally determined by independent measurements of the crystal growth rate, temperature gradient, and acetone concentration. The temperature gradient is measured by thermocouples placed on the solidification cell, the growth rate is measured directly from the motion of the sample and the acetone concentration is measured by fourier transform infrared (FT-IR) spectroscopy by calibrating the magnitude of the spectral peak at 1700-1720 cm - 1 with the concentration of acetone in the sample. Experimental results for a range of acetone concentrations and temperature gradients agree well with simple constitutional supercooling criteria and with the more exact morphological stability theory.