Abstract. This paper deals with the synthesis of combinatorial circuits by decomposition. The main idea of each decomposition method is that a complicate Boolean function is split into two or more simpler subfunctions. In this way the trivial function f = xi can be achieved after a certain number of decomposition steps and terminates the decomposition procedure. The two subfunctions of a bi-decomposition are simpler than the given function because the number of independent variables is reduced at least by one. However, for completeness, the weak bi-decomposition is needed, in which only one of the two subfunctions depends on less variables than the given function. The main aim of this paper is to answer the question whether further subfunctions for a bi-decomposition exist which depend on the same number of variables as the given function to decompose, but are simpler regarding a certain measure and allow us to determine simple circuit structures.