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].