  We present a preliminary measurement of the composition of inclusive
semileptonic B meson decays using 9.4 fb^{-1} of e^+e^- data taken with the
CLEO detector at the Upsilon(4S) resonance. In addition to measuring the
charged lepton kinematics, the neutrino four-vector is inferred using the
hermeticity of the detector. We perform a maximum likelihood fit over the full
three-dimensional differential decay distribution for the fractional
contributions from the B->X_c l nu processes with X_c = D, D^*, D^**, and
non-resonant X_c, and the process B -> X_u l nu. From the fit results we
extract <M_X2 - M_D[Bar]^2> = (0.456 +- 0.014 +- 0.045 +- 0.109) GeV^2/c^4 with
minimum lepton energy of 1.0 GeV and <M_X2 - M_D[Bar]^2> (0.293 +- 0.012 +-
0.033 +- 0.048) GeV^2/c^4 with minimum lepton energy of 1.5 GeV. The
uncertainties are from statistics, detector systematic effects, and model
dependence, respectively.
