  In the heavy quark limit, the form factors for semileptonic and rare
radiative B decays into light mesons are related by heavy quark spin symmetry.
Here we compute the leading corrections of order \Lambda/m_Q to these symmetry
relations, showing also how to include hard gluon effects to any order in
\alpha_s(m_Q). The subleading correction to the form factor relation for B\to
\pi can be computed exactly in the soft pion limit. For decays into vector
mesons, the matrix elements of two local dimension-4 operators are needed, one
of which vanishes in the constituent quark model. A few applications are
briefly discussed.
