Plagioclase, the most abundant mineral in many terrestrial and Martian igneous rocks, exhibits a range of compositions that reflects changing conditions during crystallization. Thermal emission spectrometers (TES, THEMIS, and Mini-TES) on board Mars spacecraft provide a means of determining average plagioclase compositions directly. We demonstrate that spectrally modeled average plagioclase compositions in terrestrial basalts, andesites, and dacites correspond to measured values to within +10/-6 An (molar ¿ 100) of weighted average values and +13/-14 An of normative compositions. Analyses of spectra from two-component plagioclase mixtures (varied by volume and composition) provide additional insight into potential effects of plagioclase zoning. Considering the variability in types of observed plagioclase zoning patterns in terrestrial volcanic rocks, normal zoning (calcic cores and sodic rims) cannot be assumed and thus cannot account for an apparent sodic bias seen in previous spectrally modeled compositions. This bias is better explained by higher abundances of sodic groundmass plagioclase relative to calcic phenocrysts. Deconvolution modeling of laboratory spectra of physical mixtures (convolved to various instrument resolutions) predicts that TES, THEMIS, and Mini-TES spectra may be used to extract plagioclase compositions to within +9/-12 An, +11/-12 An, and +8/12 An (respectively) of measured values in coarse sand plagioclase mixtures, with 1σ standard deviations of ¿4 An for laboratory, TES, and Mini-TES resolutions and ¿6 An for THEMIS resolution.