The first examples of carbonyl heterocubane-type clusters, [Fe(4)(μ(3)-Q)(2)(μ(3)-AsMe)(2)(CO)(12)] (2, Q = Se (a), Te (b)), which simultaneously contain elements of group 15 and 16, were obtained by thermolysis of [Fe(3)(μ(3)-Q)(μ(3)-AsMe)(CO)(9)] (1) in acetonitrile. The clusters 2 possess a cubic Fe(4)Q(2)As(2) core with alternating Fe and Q/As atoms. The coordination environment of the Fe atoms is close to octahedral, and those of Q or As atoms are tetrahedral, which determines the distorted cubic cluster core geometry. The second main products of thermolysis are the clusters [Fe(6)(μ(3)-Q)(μ(4)-Q)(μ(4)-AsMe)(2)(CO)(12)] (3a,b), whose core contains double the elemental composition of the initial cluster 1. In the case of the Se-containing cluster two other minor products [Fe(4)(μ(4)-Se)(μ(4)-SeAsMe)(CO)(12)] (4) and [Fe(3)(μ(3)-AsMe)(2)(CO)(9)] (5) are formed. Based on the structures and properties of the products, a reaction route for the conversion of 1 into 2 is proposed, which includes the associative formation of the clusters 3 as intermediates, unlike the dissociative pathways previously known for the transformations of similar clusters of the type [Fe(3)Q(2)(CO)(9)].