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 muon spin relaxation and resonance on MuSR.