The tetragonal to monoclinic (t-m) phase transition in 2.0 mol% yttria-doped tetragonal zirconia polycrystals (2Y-TZP) was studied by use of XRD and FT-IR diffuse scattering (DRIFT) measurements. The phase transition occurred isothermally when the 2Y-TZP powder after calcining at temperatures below 1200°C was annealed in air even at room temperature, and more than 90% of the monoclinic phase was obtained after a long period of annealing. However only about several percent and about 40% of the monoclinic phase were formed after the annealing treatments under vacuum and 1 atm air separately sealed in a 5 ml Pyrex tube, respectively. On the other hand, no phase transition occurred in air around room temperature in the 2Y-TZP powder after being calcined at temperatures above 1300°C. A broad band around 3300 cm^-1 due to the existence of adsorbed water was observed in the FT-IR DRIFT spectra of the 2Y-TZP powder only after calcination below 1200°C. The t-m transition behavior of 2Y-TZP powder after calcination below 1200°C differed with that after calcination above 1300°C. The existence of adsorbed water on 2Y-TZP was essential for the t-m phase transition of 2Y-TZP to take place in air around room temperature. The amount of transformed monoclinc phase was determined by the amount of adsorbed water on 2Y-TZP and supplied water from annealing atmosphere.