A large data set of single and multi-grain zircon and titanite analyses from a sample of the Oligocene Fish Canyon Tuff (FCT), a voluminous ash flow from the San Juan Mountains of Colorado and widely used Ar-40/Ar-39 geochronological standard, has been used to evaluate the influence of various sources of analytical and geological uncertainty on the calculated age of this tuft by the isotope dilution U-Pb zircon method. Twenty-three single zircon grains and seven small multi-grain fractions of the FCT yield an inverse-variance weighted mean Pb-206/U-238 date of 28.402 +/- 0.023 Ma (2-sigma, MSWD 0.93) and a slightly older weighted mean Pb-207/U-235 date of 28.529 +/- 0.030 Ma (MSWD 0.74), which are insensitive to common Pb corrections. Initial Th-230 disequilibria calculated from a newly measured Th/U = 2.2 for FCT pumice shards indicate Th-deficiency for these zircons; its correction brings the weighted mean Pb-206/U-238 date to 28.478 +/- 0.024 (MSWD 0.97), minimizing the discordance of the FCT zircon analyses. (Author)