We investigated a red to yellow color change observed in α-HgI 2 when cooled below 150 K. A phase transformation to β-HgI 2 , which has a yellow color, was ruled out by a variable temperature x-ray diffraction study. Instead the color change at cryogenic temperatures is caused by a shift in the transmission edge to shorter wavelengths which we attribute to a widening band gap at low temperatures. Using optical transmission spectroscopy the width of the band gap was measured between 10 and 330 K. At room temperature the gap was 2.11 ± 0.03 eV which is significantly smaller than the most recently published values of ∼ 2.3 eV. This smaller band gap was further verified by measuring the thermoelectric current at elevated temperatures.