  We study semileptonic decays B -> eta^(\prime) l nu, taking into account the
flavor-singlet contribution (F^{singlet}_+) to the B -> eta^(\prime) form
factors, which arises from the two-gluon emission in a decaying B meson. It has
been recently pointed out that, in addition to large weak annihilation effects,
the unknown value of F^{singlet}_+ prevents accurate theoretical estimates in
the analysis of B -> eta^\prime K decays in QCD factorization. We present a
certain method to determine F^{singlet}_+ with a reasonable accuracy, using B
-> eta^(\prime) l nu and B -> pi l nu decays. We also investigate the possible
effect of F^{singlet}_+ on the estimated branching ratios (BRs) for B ->
eta^(\prime) l nu and find that the BR for B -> eta^\prime l nu is particularly
sensitive to the effect of F^{singlet}_+.
