Reversible thermochromism in the aqueous solutions of Ln(III)=MTB chelates (Ln=14 kinds of lanthanide elements except Pm; MTB=Methylthymol blue，i. e. ，3'，3"-Bis-[N，N-(dicarboxymethyl) aminomethyl] thymolsulfonphthalein) have been investigated by spectrophotometry. The resu1ts indicated the effect of acidities and temperatures on the colors and electron spectrum of the cited systems. The results also show that the thermochromic mechanism is similar to that of the Ln(III)-Xo(Xylenol Orange) chelates systems. This is a part of the fundamental studies for developing rare earth temperature-indicating materials.