The 16-48 microns spectra of five carbon-rich post-AGB objects (pre-planetary nebulae, PPNs) known to have an unidentified 21 microns emission feature in their IRAS LRS spectra have been obtained using the Kuiper Airborne Observatory. A broad emission band extending from 24 to 45 microns is present in the spectra of these objects. Peaking at ∼30 μm, the intensity of the band is variable from source to source, accounting in one case (IRAS 22272+5435) for ∼20% of its bolometric luminosity. However, its strength is not correlated with that of the 21 microns feature. The 30 microns band is similar to the feature previously found in other carbon-rich AGB stars and planetary nebulae. It is thus seen in a wide variety of objects, while the 21 microns band is seen only in a subset of PPNs. It was suggested for AGB stars that the 30 microns band could be carried by solid MgS particles. The observed 30 microns emission is modeled combining distributions of carbon grains and MgS grains. The required MgS abundance with respect to H nuclei is roughly estimated to be less than ∼7 × 10-6, i.e., representing about 50% and 25% of the total abundances of S and Mg, respectively. Despite the relatively good fit with MgS, the possibility of alternative models especially with carbon-related compounds should still be addressed.