The effect of composition on argon retention in metamorphic hornblende has been re-evaluated by the 40 Ar 39 Ar step heating method utilizing the same samples from a K/Ar study in which a relationship between hornblende composition and argon retention was inferred. Incremental heating of the hornblendes yields 40 Ar 39 Ar age plateaus or near plateaus between 1081 ± 3 and 1112 ± 3 Ma. These data support a simple cooling history through argon closure following high-grade metamorphism, consistent with recent geochronological studies of the Bamble Sector of Norway. No clear evidence of extraneous argon is preserved in the 40 Ar 39 Ar data nor is there any sign of a thermal disturbance following argon closure. Contrary to the K/Ar results, the 40 Ar 39 Ar data show no relationship between hornblende cooling age and composition. The results of this investigation, together with those from other studies, suggest that the composition of metamorphic hornblende is not an important factor in controlling its argon closure temperature. Published studies inferring a compositional dependence on the argon closure temperature in metamorphic hornblende are commonly based on analytical data from impure, retrogressed, altered, recrystallized, or microstructurally complex hornblendes, which are not representative of the diffusive properties of a pure hornblende. Such mineralogical and structural complexities may form both above and below the closure temperature of hornblende, which may give rise to variable hornblende ages from rocks that have experienced the same geologic history. Such complexities forming at high temperature may contribute to a modified or variable characteristic grain size(s) of diffusion, resulting in a modified “complex hornblende” closure temperature. These complexities may contribute to low apparent “hornblende” ages if formed below its (true) closure temperature. In addition, variable hornblende ages may be obtained if fine-grained mineral intergrowths are present and undergo recoil-related phenomena during irradiation. Geologically meaningful hornblende ages are possible only when proper care is taken in sample selection and characterization.