The first examples of carbonyl heterocubane-type clusters, [Fe4(μ3-Q)2(μ3-AsMe)2(CO)12] (2, Q = Se (a), Te (b)), which simultaneously contain elements of group 15 and 16, were obtained by thermolysis of [Fe3(μ3-Q)(μ3-AsMe)(CO)9] (1) in acetonitrile. The clusters 2 possess a cubic Fe4Q2As2 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 [Fe6(μ3-Q)(μ4-Q)(μ4-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 [Fe4(μ4-Se)(μ4-SeAsMe)(CO)12] (4) and [Fe3(μ3-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 [Fe3Q2(CO)9].