A model of the neutral region of a planetary nebula has been constructed, building on an existing program (Abgrall et al. 1992, Astr. Astrophys. 253, 525). It incorporates a large set of equations governing the formation and destruction of molecular species and also covers photo-dissociation/ionization reactions and cosmic ray interactions. The radiation field impinging on the nebula is modelled as a 105 K diluted Planck spectrum, truncated below 91.2 nm, augmented by spectral emission lines from the ionized region (data from G. Stasinska, private communication) and the hydrogen (2s→1s) two-photon continuum. The chemical species involved in the reactions are composed of seven elements — H, He, C, O, N, S and Fe — with H and He dominating the elemental abundances. The model considers a chemical environment which is carbon-rich (i.e. C/O > 1).