Fine structure in the principal OH-stretching bands of amphiboles of the tremolite-ferro-actinolite series have been examined. In samples with partly filled A sites, a broad (composite) band is observed at 3725~680 cm−1 and is assigned to two types of configurations: (M1M1M3)-OH-A(Na,K):T1Si T1Al in which Al occurs at the T1 site, and (M1M1M3)-OH-A(Na,K)-O3(O2−,F−,Cl−); the component of (M1M1M3)-OH-A(Na,K):T1Si T1Si configuration is small, because Na and K at the A site are locally associated with Al at an adjacent T1 site. In tremolite, manganoan tremolite, and Fe2+-poor actinolite, a weak shoulder on the principal A band at ~669 cm−1 is assigned to the configuration M4CaM4(Mg,Fe2+,Mn2+,Na):(MgMgMg)-OH-A□ T1Si (□ = vacancy). Fine structure in the principal bands B (B′ and B″) and C (C′ and C″) are also observed: the higher-frequency band B″ is assigned to M1Fe2+ M1Mg M3Mg-OH-A□, and the lower-frequency band B′ to M1MgM1MgM3Fe2+-OH-A□; the higher-frequency band C′ is assigned to M1Fe2+ M1Fe2+ M3Mg-OH-A□ and the lower-frequency band C″ to M1Mg M1Fe2+ M3Fe2+-OH-A□. Some broad OH-stretching bands attributed to (M1M1M3)-OH-A□:T1Si T1Al are observed at 3640~580 cm−1. In amphiboles of the tremolite-ferro-actinolite series that show a substantial B [(MgMgFe2+)-OH] band, a new OH-stretching band (at around 3641 cm−1), E, appears near the principal C band (at around 3643 cm−1) on heat treatment. The shape of band E is similar to that of the original band B, and its local configuration is O2−-(MgMgFe3+)-OH−. A weak and broad band A** appears at ~3690 cm−1 on heat treatment of some Na-bearing actinolites, and is ascribed to the (MgMgMg)-OH-ANa-O2− configuration.