We present four new 40Ar/39Ar ages of tephra layers from an aggradational succession (Valle Giulia Formation) near the mouth of the Tiber Valley in Rome that was deposited in response to sea-level rise during Marine Isotopic Stage (MIS) 13. These new ages, integrated with seven previously determined ages, provide the only extant independent, radioisotopic age constraint on glacial termination VI and on the duration of MIS 13 sea-level rise. The new geochronologic constraints suggest a long duration for the period of sea-level rise (533 ± 2 through 498 ± 2 ka) encompassing two consecutive positive peaks of the δ18O curve (substages 13.3 and 13.1). Consistently, the litho-stratigraphic features of the sedimentary record account for two aggradational phases separated by an intervening erosional phase. Moreover, the ages obtained for this study give us the opportunity to compare the timing of the sea-level fluctuations inferred from the stratigraphic record and that provided by the astrochronologic calibration of the Oxygen isotopic curves, and to assess the calibrations of 40Ar/39Ar standards. Results of this comparison indicate that the best match is for an age of 1.186 Ma for the Alder Creek Rhyolite sanidine and 28.201 Ma for the Fish Canyon Tuff sanidine.