The decavanadate anion, [V10O28]6−, is a common constituent of vanadate minerals, and both protonated, [HxV10O28](6−x)−, and mixed-valent, [(V4+xV5+10−x)O28](6+x)−, derivatives also occur. In addition, other highly charged cations (e.g., As5+) may replace some V5+ ions. The V5+–O bond-valence equation of Brown & Altermatt (1985) gives the best calculated valences for V in decavanadates containing only V5+. The V4+–O bond-valence equation of Gagné & Hawthorne (2015) provides the best calculated valences for V4+ in non-decavanadate structures containing only V4+ in [1+4+1] coordination (i.e., one vanadyl bond, four equatorial bonds, and one longer bond trans to the vanadyl bond). Combined application of these two equations to the V–O bond lengths in decavanadates, based on V4+/V5+ estimates, provides the most reliable valence assessment. Based on the high level of agreement between valences calculated in this manner and V4+/V5+ estimates from the V5+–O bond-valence equation of Brown & Altermatt (1985), VC, the following equation gives the best estimate of the formal aggregate valence of V in decavanadate minerals: VP = 1.538(VC) – 2.692. This provides more accurate Lewis basicities for the decavanadate polyanion in analyzing its interaction with the interstitial complex. It should also help in establishing the strengths of donor-hydrogen and hydrogenacceptor bonding in decavanadate systems relevant to catalysis and bacterial-growth inhibition.