Polycyclic aromatic hydrocarbons (PAHs) and closely related materials are thought to be the carriers of the discrete infrared features at 3.3, 6.2, 7.7, 8.6, and 11.29 microns. Theory predicts that small PAHs (<20 to 30 carbon atoms) should be photochemically enriched in deuterium. These small PAHs dominate the emission intensity at the highest middle infrared frequencies, and so should emit intensely at their characteristic CD stretching frequency near 4.3 microns, a position blocked by atmospheric CO2. We propose to measure the D to H ratio in interstellar PAHs by measuring the spectra near 4.3 microns for some of the brightest PAH emission objects known. We also propose to probe the evolution of PAH D enrichment by measuring how the intensity ratio of the 4.3 micron CD stretch to the 3.3 micron PAH CH stretch varies in a series of objects which span the evolutionary range from the late carbon star/protoplanetary nebula phase, through the planetary nebula phase, into the molecular cloud phase (HII region, reflection nebula). The theory predicts that D enrichment of small PAHs will be most extensive in the older objects. If this is correct, one would expect the intensity ratio of the predicted 4.3 micron C-D band to the 3.3 micron CH band to increase in going from protoplanetary nebula, through planetary nebula, to objects embedded in molecular clouds. We now have an extensive laboratory data base on the infrared spectral properties of isolated deuterated PAHs, and will soon have one on various forms of amorphous carbon produced under D2 rich atmospheres. Thus, with ISO, we are poised to measure the first D to H ratios of objects known to show strong emission from a range of carbonaceous materials. These observations may be of relevance to Solar System studies as well.