A method of preparing an aromatic phosphine monosulfonate in zwitterionic form, such as dicyclohexylphenylphosphine monosulfonate zwitterion, involving contacting an aromatic phosphine with a sulfonating agent to provide a reaction mixture containing aromatic phosphine monosulfonate in acid form and unconverted sulfonating agent; quenching or removing substantially all of the unconverted sulfonating agent; partially neutralizing the aromatic phosphine monosulfonate in acid form to phase separate aromatic phosphine monosulfonate in zwitterionic form as a solid or neat liquid layer; and collecting the zwitterion as a solid or neat liquid. The zwitterion may be neutralized to form the corresponding aromatic phosphine monosulfonate metal salt, which is useful in preparing catalysts for hydroformylation processes.