residual entropy and an ungapped spectrum as expected for the VBG. The classical ground-state energy is highest when J$_{2}$ ≈ 2 J$_{1}$ , at the cross-over between type III ( J$_{2}$ < 2 J$_{1}$ ) and type II magnetism (the energy per spin is − J$_{1}$/ 2 + J$_{2}$/ 4 for J$_{2}$ ≤ 2 J$_{1}$ ). One possibility is that around this cross- over a spin-singlet state is energetically favored. A com- plete explanation of why spin-singlets stabilize will in the present case also involve the orbital degrees of freedom; It is the valence-bond formation rather than a Jahn- Teller effect that fixes the orbital orientation. This could be accompanied by local structural distortions which do not lead to experimentally observable [24, 29] structural changes because the valence bonds do not form a regular pattern within the crystal. As first observed by Goode- nough and Battle [21], the band width is an important factor in understanding the magnetism in double per- ovskites with 4 d and 5 d transition metal ions. Because Ba$_{2}$YMoO$_{6}$ is a relatively wide-band insulator it could also be that the stability of a spin-singet state can be understood in terms of the t − J model [31]. In conclusion, the B-site ordered double perovskite Ba$_{2}$YMoO$_{6}$ has Mo 5+ ions with a singly-occupied de- generate t$_{2}$$_{g}$ orbital in a cubic crystal field and with s = 1 / 2. These moments are located on an fcc lattice and coupled antiferromagnetically, with a Weiss temper- ature of − 160 K. At high temperature the single-ionic j = 3 / 2 moments are strongly reduced by quantum fluc- tuations, consistent with the formation of stable spin- singlet dimers at low temperatures. Remarkably, the dimerization pattern appears to be disordered, giving rise to emergent effectively unpaired spins with a diverging susceptibility as the temperature is reduced, in agree- ment with theoretical predictions of a VBG [17]. It is proposed that the stabilisation of spin-singlets is a result of the fine-tuning of the ratio J$_{2}$/J$_{1}$ to the boundary be- tween the classical type II and type III phases. Clearly the present results provide a strong motivation for fur- ther theoretical explorations of the phase diagram of the fcc antiferromagnet and of the interplay between spin and orbital degrees of freedom in 4 d and 5 d transition metal compounds. This is the first observation of a VBG in a quantum magnet. Further experimental studies are needed to understand the relationship between the struc- tural and magnetic disorder in this class of materials. The Royal Society of Edinburgh (JWGB) and Lever- hulme Trust (ACM) are acknowledged for financial sup- port. C. L. 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