  The results of our second stage (1998-2018) of the detailed spectroscopy of
peculiar supergiants identified with galactic infrared sources, performed
mainly at the 6-meter BTA telescope are summarized. The main aspect of the
program is a search for the evolutionary variations in the chemical composition
of stars, past the AGB stage and the TDU, as well as an analysis of spectral
manifestations of kinematic processes in their extended, often unstable,
atmospheres and in the envelopes. The most significant result is detection of
the s-process element excesses in seven single post-AGB stars, which confirms
the theory of evolution of this type of stars. In three of these stars we for
the first time discovered the ejection of the s-process heavy metals to the
circumstellar envelopes. A lithium excess was found in the atmospheres of two
peculiar supergiants V2324 Cyg and V4334 Sgr. The results of investigation of
the kinematical state of atmospheres and envelopes will clarify the equilibrium
of matter produced by stars in the AGB and post-AGB stages and delivered to the
interstellar medium.
