After presenting at the Alma Ata Conference our first results on the thermal decomposition of pentacarbonyl iron in zeolites of the Faujasite-type we have continued our studies by quantitatively investigating the formation of decomposition and recombination products. Our present results clearly indicate that in the first step Fe(CO)5 in zeolite X is thermally decomposed by forming zerovalent as well as divalent iron adducts. Complete desorption of CO gas yields superparamagnetic iron particles and planar-coordinated ferrous high-spin iron within the zeolite. By adding CO gas to these products (i) the iron particles react with CO forming Fe(CO)5 and other iron carbonyls as well, and (ii) the planar-coordinated iron adsorbs CO yielding spacially coordinated ferrous high-spin iron. We found these reactions to be quantitative and reversible.