Iron substitution into sphalerite, ZnS, has been studied systematically by infrared spectroscopy. A range of natural and synthetic compositions, (Zn1–xFex)S, 0 ≤ x ≤ 0.24, were examined. The IR spectrum of pure ZnS contains a single strong absorption band at 320 cm−1. With addition of FeS, the spectra become broader and shoulders appear. For compositions ≥9 mol% FeS, a splitting of the main peak occurs, and the spectra show two absorption maxima at approximately 300 and 315 cm−1, respectively. The observation of such extra features does not correspond to the usual behavior observed in other ternary mixed crystals, where either one-, two-, or mixed-mode behavior is observed. The spectra can be deconvoluted into up to three peaks, main Peaks A and B at around 300 and 315 cm−1, respectively, and a shoulder at around 330 cm−1 (Peak C). The positions and area of the peaks do not change significantly with increasing Fe content. The peak at 315 cm−1 is the main absorption peak of the host ZnS structure, and the peak at 300 cm−1 is an impurity induced mode. An effective linewidth parameter Δcorr was determined by autocorrelation analysis for each spectrum, but there are no obvious trends in the values of Δcorr that can be interpreted in terms of an inhomogeneous distribution of Fe within the sphalerite structure.