Desalting and buffer exchange are both separation processes carried out using size exclusion chromatography (SEC), also known as gel filtration chromatography. There are a few significant differences between the two processes. 

Desalting: 

• Involves removal of salts from a sample
• Is achieved by first equilibrating the chromatography column with water

Buffer Exchange: 

• Involves replacing one set of buffer salts with another
• Is accomplished by first equilibrating the column resin with the desired buffer for the sample

In both cases, the solution with which the column is pre-equilibrated replaces the buffer constituents that carry the sample into the column.