Abstract A series of S-alkyl-N-phenylthiuronium picrates, styphnates and picrolonates has been prepared and the melting points determined. By use of these derivatives it is possible to identify any of the lower primary and secondary alkyl halides examined. This has not previously been possible with S-alkylthiuronium salts. The reaction between tertiary alkyl halides and thiourea has been re-examined.