The absolute 4π–βγ coincidence counting has been used to measure the activity concentrations of 54 Mn and 203 Hg, and the Sum–Peak method was used for 125 I. 54 Mn and 125 I radionuclides have been part of international key-comparisons coordinated by the Bureau International des Poids et Measures (BIPM) in 2003/2004, while 203 Hg is part of the traceability programme of the National Laboratory for Ionizing Radiation Metrology (LNMRI)/Brazil. Three different detectors were used for the Sum–Peak method: 3″×3″ NaI(Tl) and 5″×5″ well NaI(Tl) scintillation detectors and a planar germanium detector. Direct measurements were made of the photon emission probabilities of the 35.5-, 834.8- and 279.2-keV γ-rays of 125 I, 54 Mn and 203 Hg to give values of (0.0667±0.0014), (0.9997±0.0055) and (0.8161±0.0005), respectively. The half-lives of 203 Hg and 54 Mn were also determined by means of a 4πγ ionization chamber ( 203 Hg) and by the reference source method using a HPGe detector ( 54 Mn) to give values of (46.639±0.023) days and (312.1±0.9) days, respectively.