Surface crystallization is studied in mixed alcohol-diol melts by x-ray diffraction and surface tensiometry. A reversible transition, having no bulk counterpart, from a bilayer to a monolayer surface phase, tunable by either diol concentration {phi} or temperature T , is observed. The molecular tilt is found to vary with {phi} in the bilayer phase. The structure of both surface phases is determined in detail. A simple theory, assuming a linearized {phi} dependence of the free energies of the various interfaces in each phase, accounts well for the observed ({phi},thinspT ) phase diagram. {copyright} {ital 1999} {ital The American Physical Society}