The present invention relates to a method of isolating fully thioated single stranded antisense oligonucleotides from a biological solution, which method comprises the steps of contacting the biological solution with an immobilised metal ion adsorption chromatography (IMAC) resin to adsorb the antisense oligonucleotides to the resin and subsequently contacting the resin with an eluent under conditions that provide desorption of the antisense oligonucleotides from the resin, wherein the fully thioated antisense oligonucleotides are separated from incorrectly thioated antisense oligonucleotides in the solution. The invention also relates to the use of an immobilised metal ion adsorption chromatography (IMAC) resin for isolation of fully thioated single stranded antisense oligonucleotides from a biological solution.