Astronomers have known about interstellar dust particles for almost a century, and for most of this time we have also known that these spinning grains somehow become aligned with respect to the ambient interstellar magnetic field. However, our knowledge of the grains’ composition is very incomplete, as is our understanding of the grain alignment mechanism. Such information is needed if we are to (a) remove the contaminating dust signal from cosmological searches for gravity waves from the inflationary epoch, and (b) efficiently exploit polarimetry of aligned grains to study the role of magnetic fields in star/planet formation. By describing two recent observations, I will show how measurements of the polarization spectrum of dust emission can help fill in the gaps in our knowledge of grain composition and alignment. These two observations are (1) the first measurement of the submillimeter polarization spectrum of a translucent molecular cloud, that my collaborators and I obtained with a balloon-borne experiment flown over Antarctica, and (2) the first observation of a far-IR polarization spectrum that rises with increasing wavelength as predicted by models, made toward the nearby stellar nursery Rho Oph using the new polarimeter for the Stratospheric Observatory For Infrared Astronomy (SOFIA).