The water vapor adsorption isotherms of amaranth starch were determined for the temperature range 25-50 degrees C and water activity 0.04-0.90. Comparison with adsorption isotherms of wheat and corn native starches showed that the three starches have similar adsorption capacities at low a(w) values. Amaranth starch showed higher sorption capacity than corn and wheat starches at high a(w) values. Two BET-like isotherm equations were used to fit the equilibrium data: the GAB isotherm and a fractal isotherm. Although the best fit was obtained with fractal isotherm, the values of C and m(m) obtained from both equations were practically coincident. The fractal dimension D was in average 2.77 revealing the high degree of surface roughness of the starch. The effect of temperature on the equilibrium data was satisfactorily modeled by means of a simple two-parameter equation, covering the whole a, range investigated. An analytical expression was used to estimate the variation of the isosteric heat with moisture. The isosteric heat reached its maximum value of 30 kJ/mol at moisture levels of about 5% (db).