For gaseous hydrogen isotopes at temperatures between 22 K and 110 K, measurements are reported on transverse heat flow, which occurs under the influence of a magnetic field. For HD, the effect is found to be much larger than for H 2 and D 2. The observed strong temperature dependence of the effect is compared with theoretical expressions. Mixtures of para hydrogen with ortho hydrogen as well as mixtures of ortho deuterium with helium have also been investigated.