It has been suggested that renewable energy sources be considered as replacements or diluents for middle distillate ground transportation fuels. It is vital for the operational considerations of these fuels to investigate the many positives and negatives that this use could prompt. In proposing such a replacement, considerations must be given to the many problems that could arise. Problems to be studied include fuel storage stability, fuel solubility, and oxidative stability. Unlike newly manufactured soy oils, it was found that this recycled soy oil was not stable in fuels. The question was, what in the recycled oil led to the observed fuel degradation.