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.