Ambient-cured epoxies used to bond high strength continuous fibers to concrete for strengthening purposes can have glass transition temperatures (Tg) that are not only close to the service temperature but can also change over time as a function of environmental conditions. While the design ramifications of using such an epoxy resin with an evolving Tg close to the service temperature are unclear and difficult to predict, recommended methods for assigning a Tg for such resins are nonetheless needed. In the present investigation, an interlaboratory evaluation of Tg characteristics of several ambient-cured epoxy resins using dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) was carried out. The need for standardized specimen conditioning prior to Tg measurement is demonstrated. Of the three DMA Tg methods employed, the tan δ method provided the lowest standard deviation. Because of its reliability and its roots in mechanical testing, the DMA tan δ method applied during a single temperature up-ramp is recommended for assigning Tg.