Abstract:A graph property is any class of simple graphs, which is closed under isomorphisms. Let H be a given graph on vertices v 1 , . . . , v n . For graph properties P 1 , . . . , P n , we denote by H[P 1 , . . . , P n ] the class of those (P 1 , . . . , P n )-partitionable graphs G, with a corresponding vertex partition (V 1 , . . . , V n ), for which an edge {x i , x j } with x i ∈ V i and x j ∈ V j implies the existence of the edge {v i , v j } in the graph H. The problem of the unique description of a graph property P in the form H[P 1 , . . . , P n ] is investigated for P, P 1 , . . . , P n being from the class L a of all graph properties closed under taking disjoint unions and subgraphs. The unique factorization theorems obtained in the paper generalize known results of this type bringing together •-reducibility over L a and ∨-reducibility in the lattice (L a , ⊆). There is also offered a new insight into the modular decomposition tree for a graph.