Abstract. We give a sufficient condition on a function f : R k → R so that it takes by superposition the homogeneous vector valued spaceẆ m p ∩ W 1 mp (R n , R k ) into the corresponding real valued space, for integers m, n, k such that m ≥ 2, k, n ≥ 1, and p ∈ [1, +∞[. In case k = 1, this condition also turns out to be necessary. For k > 1, it is not proved to be necessary, but it is weaker than the conditions used till now, such as the continuity and boundedness of all derivatives up to order m.