We report Spitzer/IRS spectra of a small sample of heavily obscured and high mass loss post-AGB stars with O-rich maser emission. IRAM 30 m observations for two post-AGBs are also reported. The Spitzer/IRS spectra show the C-rich character of their circumstellar envelopes (CSE), as revealed by the presence of small hydrocarbon molecules such as C2H2, C4H2, C6H2, C6H6 , and HCN. Benzene (C6H6) is detected toward two sources in our sample, bringing up to three the total number of Galactic post-AGB objects where this molecule has been detected. The available IRAM 30 m data confirm that the central stars are C-rich, despite the presence of O-rich masers, and the presence of high velocity molecular outflows together with extreme mass loss rates (10‑4 M⊙ yr‑1). Our Spitzer and IRAM 30 m observations confirm the model of Cernicharo (2004) that predicts a rich photochemistry in the neutral regions of these objects on timescales shorter than the dynamical evolution of the central HII region, leading to the formation of large, complex carbon-rich molecules. This model also explain the presence of O-rich masers in C-rich post-AGB stars as a consequence of the extreme mass loss rate, being not necessary to invoke a chemistry change in their circumstellar envelopes.