Most oxyanions are weak bases and can be protonated to give acids or acid salts. For example, the phosphate ion can be successively protonated to form phosphoric acid.
The extent of protonation in aqueous solution will depend on the acid dissociation constants and pH. For example, AMP (adenosine monophosphate) has a pK value of 6.21, so at pH 7 it will be about 10% protonated. Charge neutralization is an important factor in these protonation reactions. By contrast, the univalent anions perchlorate and permanganate ions are very difficult to protonate and so the corresponding acids are strong acids.
Although acids such as phosphoric acid are written as , the protons are attached to oxygen atoms forming hydroxyl groups, so the formula can also be written as  to better reflect the structure. Sulfuric acid may be written as ; this is the molecule observed in the gas phase.
The phosphite ion, , is a strong base, and so always carries at least one proton. In this case the proton is attached directly to the phosphorus atom with the structure . In forming this ion, the phosphite ion is behaving as a Lewis base and donating a pair of electrons to the Lewis acid, .
As mentioned above, a condensation reaction is also an acid–base reaction. In many systems, both protonation and condensation reactions can occur. The case of the chromate ion provides a relatively simple example. In the predominance diagram for chromate, shown at the right, pCr stands for the negative logarithm of the chromium concentration and pH stands for the negative logarithm of  ion concentration. There are two independent equilibria. Equilibrium constants are defined as follows.
The predominance diagram is interpreted as follows. 
*The chromate ion, , is the predominant species at high pH.  As pH rises the chromate ion becomes ever more predominant, until it is the only species in solutions with pH > 6.75.
*At pH  the hydrogen chromate ion,  is predominant in dilute solution.
*The dichromate ion, , is predominant in more concentrated solutions, except at high pH.
The species  and   are not shown as they are formed only at very low pH. 
Predominance diagrams can become very complicated when many polymeric species can be formed, such as in vanadates, molybdates, and tungstates. Another complication is that many of the higher polymers are formed extremely slowly, such that equilibrium may not be attained even in months, leading to possible errors in the equilibrium constants and the predominance diagram.