Regional Research Laboratory, Bhubaneswar-751 013, India Manuscript received 4 July 2003, revised 9 March 2004, accepted 10 March 2004 An attempt has been made to study the thermal dehydration of FeCI2.4H2O in static air as well as in flowing nitrogen atmosphere using the simultaneous DTA/TG method. In static air or self-generated atmosphere, the salt dehydrates in three steps losing about 2.0, 1.1 and 1.0 moles of H2O, respectively which do not undergo any significant change with increase in heating rate from 2° to 10°/min. In flowing nitrogen atmosphere dehydration also takes place in three steps. While the average loss of 2.1 moles of H2O in the first step does not show any variation with heating rate, loss in the second step tends to increase with corresponding decrease in the third step with increase in heating rate. Consequently, the total loss of H2O in nitrogen atmosphere in about 0.5 moles higher than in the static air. While in static air nucleation and growth appears to be the most appropriate mechanistic model, in flowing nitrogen only the first step obeys this model. The other two steps obey the contracting phase boundary model. A comparison of the thermal dehydration characteristics between MnCl2 and FeCI2 hydrates has been presented.