Some primitive meteorites and interplanetary dust particles (IDPs) exhibit large excesses in deuterium (D) and/or 15N relative to terrestrial values. These anomalies likely represent the partial preservation of materials that experienced extreme chemical mass fractionation in the cold, dense molecular cloud predating our Solar System. The largest D/H ratios observed so far in extraterrestrial materials occur in IDPs, reaching the values of some molecules in interstellar molecular clouds. Constraints on the nature of the D- and 15N-rich carrier phases in IDPs and meteorites are reviewed.