Abstract. Let X Y and Z be Banach function spaces over a measure space (Ω, Σ, µ). Consider the spaces of multiplication operators X Y from X into the Köthe dual Y of Y , and the spaces X Z and Z Y defined in the same way. In this paper we introduce the notion of factorization norm as a norm on the product space X Z · Z Y ⊆ X Y that is defined from some particular factorization scheme related to Z. In this framework, a strong factorization theorem for multiplication operators is an equality between product spaces with different factorization norms. Lozanovskii, Reisner and Maurey-Rosenthal theorems are considered in our arguments to provide examples and tools for assuring some requirements. We analyze the class d * p,Z of factorization norms, proving some factorization theorems for them when p-convexity/p-concavity type properties of the spaces involved are assumed. Some applications in the setting of the product spaces are given.Mathematics Subject Classification (2010). Primary 46E30; Secondary 47B38, 46B42, 46B28.