A process is described for the determination of soluble aldehydes in a composition including one or more of (a) 2,5-furandicarboxylic acid (FDCA), (b) terephthalic acid (TPA), (c) an ester of 2,5-furandicarboxylic acid and (d) an ester of terephthalic acid. The process can indicate the presence in the composition of even those very low levels of such soluble aldehydes that have been associated with the development of unacceptable color in a) the composition, b) some portion of the composition or c) a prepolymer, oligomer or polymer prepared, directly or indirectly, at least in part from the composition or some portion of the composition, so that mitigating or corrective measures can be undertaken in response.