Recent ground-based abundance studies of a large sample of PN and H II regions by the applicants have revealed that the ratio of C, N and O abundances derived from optical recombination lines to those from optical and UV collisionally excited forbidden lines, covers a wide range of 1--15. Since this ratio varies from object to object the cause has to be sought in the physical conditions in the nebulae, not in the basic atomic physics. A possible explanation is that significant temperature and/or density fluctuations exist. The far-IR fine structure lines from [N III], [O III], [Ne III], [S III] and [Ar III], observable with the ISO SWS and LWS, provide a unique and powerful tool to solve this discrepancy in nebular abundance determinations. With excitation energies of less than 1100 K, ionic abundances derived from these far-IR lines are insensitive to the electron temperature and to temperature fluctuations and provide an independent test of the reliability of the abundances derived from optical recombination lines on the one hand and from optical and UV forbidden lines on the other. The availability from these far-IR lines of a number of density-sensitive diagnostic ratios with a wide range of critical densities will enable us to study quantitatively nebular density inhomogeneity and its effects on abundance determinations, without complications from temperature or ionization stratification. ISO LWS01 full grating spectra for about ten PN suitable for such abundance studies are scheduled in our LWS Post-Main-Sequence GT programme. SWS01 spectra for a few of them will also be obtained by us, with most of the remainder being observed by the SWS consortium. Here we propose to obtain LWS01 and SWS01 full grating spectra from 2.43--196.7 um for an additional sample of 32 PN, selected based on fresh information from our recent extensive ground-based abundance survey. A systematic study of a large enough sample of PN having different morphologies, electron temperatures and densities, and exciting stars, is essential to achieve a full understanding of this fundamental discrepancy in nebular abundance determinations from heavy element optical forbidden and recombination lines, which has a major impact on our understanding of stellar nucleosynthesis and Galactic chemical evolution.