The effects of solution speciation on the bioavailability of trace metals are well documented, but the role of speciation in the bioavailability of oxyanionic trace elements that may form significant ion pairs with Ca and Mg in saline media has not been investigated. We assessed the effects of such ion pairing on the availability of selenate to representative monocotyledonous and dicotyledonous higher plants. Formation constants for the and ion pairs were experimentally determined using 0 0
CaSO CaSeO 4 4
a Ca 2Ο© ion-selective electrode. The published value of 10 2.3 for formation was confirmed, but the value of 10 2.7 for 0 CaSO 4 was found to be in error; a value of 10 2.0 is proposed here as the correct formation constant. Five solution culture experiments 0 CaSeO 4 were conducted using alfalfa (Medicago sativa L.) or tall wheatgrass (Elytrigia pontica [Podp.] Holub) with treatments consisting of NaSeO 4 levels in combination with various levels of MgCl 2 or CaCl 2 . Both shoot Se concentrations and whole-plant Se contents were highly correlated with the free activity but were poorly correlated with the sum of the free ion plus Ca and Mg ion 2Οͺ SeO 4 pair species. Thus, we have shown, for the first time, that the free ion model of trace metal bioavailability is also valid for oxyanions that form complexes with Ca and Mg in saline media but that this conclusion hinges critically on the accuracy of the pertinent formation constants.