Two samples of Triton X-100 (TX100) were compared. The "old sample," manufactured by Rohm and Haas, has a mean degree of ethoxylation of 10.1. The "new sample," manufactured by Union Carbide, has a mean degree of ethoxylation of 9.6. The cmc values near room temperature (23 degrees C, old sample; 24.6 degrees C, new sample), as determined from surface tension measurements, are similar. Dynamic light scattering measurements on the new sample gave hydrodynamic radii at different temperatures in reasonable accord with those reported a number of years ago. In contrast, time-resolved fluorescence quenching (TRFQ) experiments using a pyrene-labeled triglyceride as a probe gave aggregation numbers N(agg) near room temperature that were similar to those determined previously by light scattering; however, the N(agg) of the new sample was larger, and the difference was outside of experimental error to one standard deviation. More troubling was that the TRFQ experiment failed to give meaningful results on solutions exposed to air, as in the case of the older TX100 sample. To get useful data by TRFQ measurements, one had to first remove oxygen from the solution. Copyright 2000 Academic Press.