  The phase diagram of a one-dimensional tight-binding model with a
pair-hopping term (amplitude V) has been the subject of some controvery. Using
two-loop renormalization group equations and the density matrix renormalization
group with lengths L<=60, we argue that no spin-gap transition occurs at
half-filling for positive V, contrary to recent claims. However, we point out
that away from half-filling, a *phase-separation* transition occurs at finite
V. This transition and the spin-gap transition occuring at half-filling and
*negative* V are analyzed numerically.
