We propose the determination of the CKM matrix element

| V_{ub} |

by the measurement of the spectrum of

\vec{B} D_{s}^{+} X_{u},

dominated by the spectator quark model mechanism

\bar{b} \to D_{s}^{(*) +} ū .

The interest in considering

\vec{B} D_{s}^{+} X_{u}

versus the semileptonic decay is that more than 50% of the spectrum for

\vec{B} D_{s}^{+} X_{u}

occurs above the kinematical limit for

\vec{B} D_{s}^{+} X_{c},

while most of the spectrum

\vec{B} l ν X_{u}

occurs below the

\vec{B} l ν X_{c}

one. Furthermore, the measure of the hadronic mass

M_{X}

is easier in the presence of an identified

D_{s}

than when a

ν

has been produced. As a consistency check, we point out that the rate

\bar{b} \to D_{s}^{(*) +} \bar{c}

(including QCD corrections) is consistent with the measured

BR (\vec{B} D_{s}^{\pm} X)

by CLEO. Although the hadronic complications may be more severe in the decay mode that we propose than in the semileptonic inclusive decay, the end of the spectrum in

\vec{B} l ν X_{u}

is not well understood on theoretical grounds. We argue that, in our case, the excited mesons

D_{s}^{* *},

decaying into

D K,

do not contribute and, if there is tagging of the B meson, the other mechanisms to produce a

D_{s}

of the right sign are presumably small, of

{O (10}^{- 2})

relative to the spectator amplitude, or can be controlled by kinematical cuts. We discuss in detail the hadronic uncertainties of the method and present an error calculation on

| V_{qb} |

(q = u, c)

and on the ratio

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