  We examine a periodic mixed spin chain with spin magnitudes 1/2 and 1 which
are arrayed as 1/2-1/2-1-1. The three independent parameters are ratios of the
four exchange couplings. We determine phase boundaries in the parameter space
by using the gapless condition which was previously derived by mapping a
general inhomogeneous spin chain to the nonlinear sigma model. We find two
gapless boundaries separating three disordered phases. The features of the
phases are explained in terms of singlet clusters.
