In the present work we analyze an extensive active region spectrum observed by the SUMER instrument on board SOHO with the aim of determining the thermal structure of the emitting plasma. We found that the plasma is made of three distinct, isothermal components, whose physical properties are similar to coronal hole, quiet-Sun, and active region plasmas. The temperatures of the coronal hole-like and quiet-Sun-like plasmas are in excellent agreement with previous measurements obtained outside active regions. We also used a DEM diagnostic technique to check the robustness of our results and found that the DEM curves are compatible with the presence of three distinct nearly isothermal plasmas if the individual DEM measurements are smoothed over a small temperature interval. Larger intervals lead the resulting DEM curves to a more multithermal behavior, raising the question of whether multithermal active region DEM curves available in the literature are real or an artifact of oversmoothing. The results are compared with measurements of the temperature of individual loop structures in the literature and discussed in light of a new picture of the solar corona.