  The binodal of a boson square-well fluid is determined as a function of the
particle mass through the newly devised quantum Gibbs ensemble Monte Carlo
algorithm [R. Fantoni and S. Moroni, {\sl to be published}]. In the infinite
mass limit we recover the classical result. As the particle mass decreases the
gas-liquid critical point moves at lower temperatures. We explicitely study the
case of a quantum delocalization de Boer parameter close to the one of
$\mbox{}^4$He. For comparison we also determine the gas-liquid coexistence
curve of $\mbox{}^4$He for which we are able to observe the binodal anomaly
below the $\lambda$-transition temperature.
