The complexation of americium(III) with nitrate was studied at temperatures from 10 to 85 °C in 1 M HNO₃–HClO₄ by spectrophotometry. The 1 : 1 complex species, AmNO₃²⁺, was identified and the stability constants were calculated from the absorption spectra recorded for titrations at several temperatures. Specific ion interaction theory (SIT) was used for ionic strength corrections to obtain the stability constants of AmNO₃²⁺ at infinite dilution and variable temperatures. The absorption spectra of Am(III) in diluted HClO₄ were also reviewed, and the molar absorptivity of Am(III) at around 503 nm and 813 nm was re-calibrated by titrations with standardized DTPA solutions to determine the concentration of Am(III).