The most common phosphonium compounds have four organic substituents attached to phosphorus. The quaternary phosphonium cations include tetraphenylphosphonium, (CH)P and tetramethylphosphonium .
Quaternary phosphonium cations () are produced by alkylation of organophosphines. For example, the reaction of triphenylphosphine with methyl bromide gives methyltriphenylphosphonium bromide:
:PPh + CHBr → [CHPPh]Br
The methyl group in such phosphonium salts is mildly acidic, with a pK estimated to be near 15:
:[CHPPh] + base → CH=PPh +  [Hbase]
This deprotonation reaction gives Wittig reagents.