We present a detailed analysis of the mid-infrared spectrum of the peculiar evolved object HR 4049 . The full Spitzer-IRS high-resolution spectrum shows a wealth of emission with prominent features from CO _ { 2 } and H _ { 2 } O and possible contributions from HCN and OH . We model the molecular emission and find that it originates from a massive ( M \gtrsim 8 \times 10 ^ { -3 } M _ { \sun } ) , warm ( T _ { ex } \approx 500 K ) and radially extended gas disk that is optically thick at infrared wavelengths . We also report less enrichment in ^ { 17 } O and ^ { 18 } O than previously found and a comparison of the Spitzer observations to earlier data obtained by ISO-SWS reveals that the CO _ { 2 } flux has more than doubled in 10 years time , indicating active and ongoing chemical evolution in the circumbinary disk . If the gas originates from interaction between the stellar wind and the dust , this suggests that the dust could be oxygen-rich in nature . The molecular gas plays a crucial role in the thermal properties of the circumbinary disk by allowing visible light to heat the dust and then trapping the infrared photons emitted by the dust . This results in higher temperatures and a more homogeneous temperature structure in the disk .