ABSTRACT+ transitions observed are consistent with this ion arising from denser, inner (and presumably warmer) layers of the fossil remnant of the slow AGB CSE at the core of the nebula. From rotational diagram analysis, we deduce excitation temperatures of T ex ∼ 10−20 K for all ions except for H 3 O + , which is most consistent with T ex ≈ 100 K. Although uncertain, the higher excitation temperature suspected for H 3 O + is similar to that recently found for H 2 O and a few other molecules, which selectively trace a previously unidentified, warm nebular component. The column densities of the molecular ions reported here are in the range N tot ≈ [1−8] × 10 13 cm −2 , leading to beam-averaged fractional abundances relative to H 2 of X(HCO + ) ≈ 10 −8 , X(H 13 CO + ) ≈ 2 × 10 −9 , X(SO + ) ≈ 4 × 10 −9 , X(N 2 H + ) ≈ 2 × 10 −9 , and X(H 3 O + ) ≈ 7 × 10 −9 cm −2 . We have performed chemical kinetics models to investigate the formation of these ions in OH 231.8 as the result of standard gas phase reactions initiated by cosmic-ray and UV-photon ionization. The model predicts that HCO + , SO + , and H 3 O + can form with abundances comparable to the observed average values in the external layers of the slow central core (at ∼[3−8] × 10 16 cm); H 3 O + would also form quite abundantly in regions closer to the center (X(H 3 O + ) ∼ 10 −9 at ∼10 16 cm). For N 2 H + , the model abundance is lower than the observed value by more than two orders of magnitude. The model fails to reproduce the abundance enrichment of HCO + , SO + , and N 2 H + in the lobes, which is directly inferred from the broad emission profiles of these ions. Also, in disagreement with the narrow H 3 O + spectra, the model predicts that this ion should form in relatively large, detectable amounts (≈10 −9 ) in the external layers of the slow central core and in the high-velocity lobes. Some of the model-data discrepancies are reduced, but not suppressed, by lowering the water content and enhancing the elemental nitrogen abundance in the envelope. The remarkable chemistry of OH 231.8 probably reflects the molecular regeneration process within its envelope after the passage of fast shocks that accelerated and dissociated molecules in the AGB wind ∼800 yr ago.