The toroidal interferometer and polarimeter (TIP) is one of the primary electron density diagnostics on ITER. To meet measurement requirements over several thousand seconds, environmental factors such as changes in the window temperature, air temperature, and humidity, which can cause uncompensated phase drifts, must be minimized. This paper reports measurements of the phase shift induced by ZnSe and BaF2 windows as their temperature is varied utilizing the TIP prototype. To accomplish these measurements, test pieces of BaF2 and ZnSe are placed in a small oven that is located in one leg of the 10.59/5.22 µm two-color interferometer TIP prototype. The oven temperature was varied from room temperature to ∼90 °C, which is slightly higher than the expected window temperatures on ITER. The vibration compensated phase shifts measured for these two materials are dϕvc/dT/L = 0.0121 and −0.275 deg. °C−1 cm−1 for BaF2 and ZnSe, respectively. From these measurements, it was concluded that temperature variations of the environmental/secondary confinement barrier window and primary vacuum window have to be suppressed within approximately 1 °C and 2 °C, respectively, in order to passively meet the electron density measurement requirements in ITER.