In this work we use infrared spectra recorded by the Infrared Atmospheric Sounding Interferometer (IASI) to characterize the emissions from the Mount Kasatochi volcanic eruption on 7 and 8 August 2008. We first derive the total atmospheric load of sulfur dioxide (SO₂) and its evolution over time. For the initial plume, we found values over 1.7 Tg of SO₂, making it the largest eruption since the 1991 eruptions of Pinatubo and Hudson. Vertical profiles were retrieved using a line-by-line radiative transfer model and an inversion procedure based on the optimal estimation method (OEM). For the Kasatochi eruption, we found a plume altitude of 12.5 ± 4 km. Taking advantage of IASI's broad spectral coverage, we used the ν₃ band (∼1362 cm⁻¹) and, for the first time, the ν₁ + ν₃ band (∼2500 cm⁻¹) of SO₂ for the retrievals. While the ν₃ band saturates easily for high SO₂ concentrations, preventing accurate retrieval, the ν₁ + ν₃ band has a much higher saturation threshold. We also analyzed the broadband signature observed in the radiance spectra in the 1072–1215 cm⁻¹ range associated with the presence of aerosols. In the initial volcanic plume the signature matches closely that of mineral ash, while by 10 August most mineral ash is undetectable, and the extinction is shown to match closely the absorption spectrum of liquid H₂SO₄ drops. The extinction by sulphuric acid particles was confirmed by comparing spectra before and a month after the eruption, providing the first spectral detection of such aerosols from nadir view radiance data.