  We present IRAM Plateau de Bure Interferometer observations of the 12CO(3-2)
emission from two far-infrared luminous QSOs at z ~ 2.5 selected from the
Herschel-ATLAS survey. These far-infrared bright QSOs were selected to have
supermassive black holes (SMBHs) with masses similar to those thought to reside
in sub-millimetre galaxies (SMGs) at z ~ 2.5; making them ideal candidates as
systems in transition from an ultraluminous infrared galaxy phase to a sub-mm
faint, unobscured, QSO. We detect 12CO(3-2) emission from both QSOs and we
compare their baryonic, dynamical and SMBH masses to those of SMGs at the same
epoch. We find that these far-infrared bright QSOs have similar dynamical but
lower gas masses than SMGs. In particular we find that far-infrared bright QSOs
have ~50+-23% less warm/dense gas than SMGs, which combined with previous
results showing the QSOs lack the extended, cool reservoir of gas seen in SMGs,
suggests that they are at a different evolutionary stage. This is consistent
with the hypothesis that far-infrared bright QSOs represent a short (~1Myr) but
ubiquitous phase in the transformation of dust obscured, gas-rich,
starburst-dominated SMGs into unobscured, gas-poor, QSOs.
