  Quintessence has been introduced as an alternative to the cosmological
constant scenario to account for the current acceleration of the universe. This
new dark energy component allows values of the equation of state parameter
$w_{Q}^0\geq-1$, and in principle measurements of cosmological distances to
Type Ia supernovae can be used to distinguish between these two types of
models. Assuming a flat universe, we use the supernovae data and measurements
of the position of the acoustic peaks in the Cosmic Microwave Background (CMB)
spectra to constrain a rather general class of Quintessence potentials,
including inverse power law models and recently proposed Supergravity inspired
potentials. In particular we use a likelihood analysis, marginalizing over the
dark energy density $\Omega_{Q}$, the physical baryon density $\Omega_{b}h^2$
and the scalar spectral index $n$, to constrain the slopes of our Quintessence
potential. Considering only the first Doppler peak the best fit in our range of
models gives $w_{Q}^0\sim-0.8$. However, including the SnIa data and the three
peaks, we find an upper limit on the present value of the equation of state
parameter, $-1\leq w_{Q}^0\leq-0.93$ at $2\sigma$, a result that appears to
rule out a class of recently proposed potentials.
