Two new selective reagents in the scries of nitrophonylhydrazones of vicinal diketornonoximes, F-nitro-and 2 : 4, dinitro-phenylhydrazones of 2-methyl-3 oximo-pentanone-4 are repeated. Both show high sensitivity and selectivity for cobalt. Limits of identification and dilution limits for the reagents have been determined as 0.2 microgram and 1 : 500,000 respectively.