Temperature is a key parameter controlling the rheology of lava flows. However, the unpredictable hazard of eruptions prevents direct measurements of hot volcanic bodies. Thus, the temperature of lava products is mostly retrieved by remote sensing techniques (ground- or satellite-based detectors) build on measuring infrared (IR) radiance. These well-established techniques are however subjected to important errors related to the poor knowledge of one of the most critical parameters, namely spectral emissivity. The aim of our study is to fill this gap by exploring the emissivity–temperature relationship of three different basalts. We performed in situ emissivity measurements at high temperatures (up to 1800 K) over a wide spectral range (350–8000 cm−1; 1.25–28.6 μm), using a non-contact IR apparatus. To unravel the complex radiative behavior of the samples with temperature, structural, chemical and textural analyses using Raman spectroscopy, XRD, DSC, SEM, EMPA and TEM were systematically performed. Our results show that spectral emissivity varies with temperature, wavenumber, and the sample degree of polymerization combined with total FeO content. Spectral emissivity is greatly affected by the crystallization of Fe-Mg-rich crystals at the micro-scale. Consequently, this study proves that spectral emissivity cannot be assumed constant for a single magmatic composition. Finally, our laboratory-measured values of spectral emissivity allowed refining the remote sensing temperature of the 2014–2015 Holuhraun eruption (Bárðarbunga, volcano) by 50 K in average. These new insights will ultimately reduce uncertainties in temperature estimates based on IR technology, information that are crucial to improve hazard assessment in volcanic crisis.