We have discovered an unusual magnetovolume effect in the 6H-perovskite oxides Ba3BiIr2O9 and Ba3BiRu2O9. These compounds contain isolated face-sharing bi-octahedral dimers Ir2O9 (Ru2O9) which permit direct Ir-Ir (Ru-Ru) bonding. The iridium compound shows an extremely rapid, large (1%) and anisotropic volume increase on cooling through a critical temperature T = 72 K. The ruthenium compound shows an analogous, but weaker and less sharp, effect at 170 K. The volume increase is driven by a 5% increase in the Ir¿Ir bond length at T, and is accompanied by a sharp drop in magnetic susceptibility. The most likely explanation is the opening of a spin-gap, where the observed rapid decrease in susceptibility is related to the formation of local spin singlets (magnetic dimers). We wish to investigate this phenomenon using inelastic neutron scattering on Merlin.