The present invention provides method of increasing the percentage of monomer in a composition of recombinantly expressed antibody molecules characterised in that the antibody molecule comprises at least one Fv with specificity for an antigen of interest comprising one VH and one VL wherein said VH and VL are connected directly or indirectly via one or more linkers and are stabilised by a disulfide bond therebetween, said method comprises: a) a conversion step of treating the composition with a denaturant selected from urea and/or Guanidine hydrochloride; b) wherein step a) is performed in the presence of a reducing agent or after treatment with a reducing agent.