  We present results from a set of simulations using a fully coupled
three-dimensional (3D) chemistry-radiation-hydrodynamics model and investigate
the effect of transport of chemical species by the large-scale atmospheric flow
in hot Jupiter atmospheres. We couple a flexible chemical kinetics scheme to
the Met Office Unified Model which enables the study of the interaction of
chemistry, radiative transfer and fluid dynamics. We use a newly-released
"reduced" chemical network comprising 30 chemical species that has been
specifically developed for application in 3D atmosphere models. We simulate the
atmospheres of the well-studied hot Jupiters HD~209458b and HD~189733b which
both have dayside--nightside temperature contrasts of several hundred Kelvin
and superrotating equatorial jets. We find qualitatively quite different
chemical structures between the two planets, particularly for methane (CH$_4$),
when advection of chemical species is included. Our results show that
consideration of 3D chemical transport is vital in understanding the chemical
composition of hot Jupiter atmospheres. 3D mixing leads to significant changes
in the abundances of absorbing gas-phase species compared with what would be
expected by assuming local chemical equilibrium, or from models including 1D -
and even 2D - chemical mixing. We find that CH$_4$, carbon dioxide (CO$_2$) and
ammonia (NH$_3$) are particularly interesting as 3D mixing of these species
leads to prominent signatures of out-of-equilibrium chemistry in the
transmission and emission spectra, detectable with near-future instruments.
