An IR achromatic retarder was aligned and characterized using the University of Alabama in Huntsville's Fourier transform IR spectropolarimeter. The FTIR spectropolarimeter produces a full polarization description of a sample over wavelengths 3-14 micrometers . Mueller matrices were measured for different relative alignments between the complementary plates of the achromat until the retardance orientation variation was reduced to within +/- 1 degree and the retardance magnitude varied smoothly with a peak-to-valley difference of 24 degrees from 4-14 micrometers . The results presented here include the progression of retardance magnitudes and retardance orientations as the plates alignment varied as well as the final Mueller matrix and retardance components of the achromat element.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.