  In the low dimensional spin systems $SrCu_2(BO_3)_2$ and $CuGeO_3$ the
thermal conductivities along different crystal directions show pronounced
double-peak structures and strongly depend on magnetic fields. For
$SrCu_2(BO_3)_2$ the experimental data can be described by a purely phononic
heat current and resonant scattering of phonons by magnetic excitations. A
similar effect seems to be important in $CuGeO_3$, too but, in addition, a
magnetic contribution to the heat transport may be present.
