Among the hot Jupiters that transit their parent stars known to date, the two best 
candidates to be observed with transmission spectroscopy in the mid-infrared (MIR) are 
HD189733b and HD209458b, due to their combined characteristics of planetary density, orbital 
parameters and parent star distance and brightness. Here we present simulations of 
transmission spectra of these two planets during their primary eclipse in the MIR, and 
sensitivity studies of the spectra to the changes of atmospheric thermal properties, 
molecular abundances and C/O ratios.
Our model predicts that the dominant species absorbing in the MIR on hot Jupiters are water 
vapor and carbon monoxide, and their relative abundances are determined by the C/O ratio. 
Since the temperature profile plays a secondary role in the transmission spectra of hot 
Jupiters compared to molecular abundances, primary eclipse observations in the MIR of those 
objects can give an insight on EGP atmospheric  chemistry. We discuss our theoretical 
results in light of the recent observations we made with the Spitzer Space telescope.