The prefix thio-, when applied to a chemical, such as an ion, means that an oxygen atom in the compound has been replaced by a sulfur atom. This term is often used in organic chemistry. For example, the word ether refers to an oxygen-containing compound having the general chemical structure R-O-R', where R and R' are organic residues and O is an oxygen atom.  Thioether refers to an analogous compound with the general structure R-S-R' where S is a sulfur atom covalently bonded to two organic residues. A chemical reaction involving the replacement of oxygen to sulfur is called thionation or thiation.

The prefix thio-, when applied to a chemical, such as an ion, means that an oxygen atom in the compound has been replaced by a sulfur atom. This term is often used in organic chemistry. For example, the word ether refers to an oxygen-containing compound having the general chemical structure R-O-R', where R and R' are organic residues and O is an oxygen atom.  Thioether refers to an analogous compound with the general structure R-S-R' where S is a sulfur atom covalently bonded to two organic residues. A chemical reaction involving the replacement of oxygen to sulfur is called thionation or thiation.