We report a new method for synthesizing a composition superlattice with molecular beam epitaxy. A repeated structure of InAs(l-x)Sb(x)/InAs(1-y)Sb(y) was induced by periodic cycling of a non-incorporating Bi-flux, while keeping the In-, As- and Sb-fluxes constant. The Bi acts as a virtual chemical valve. Highresolution X-ray diffraction demonstrated a modulation ratio of x/y=1:1.3. Composition broadening could not be detected within the experimental resolution. The Bi, despite not incorporating, also does not remain on the growing surface after the incoming Bi-flux is terminated. Published by Elsevier B.V.