A system and method for distributing one or more gases to an atomic layer deposition (ALD) reactor. An integrated inlet manifold block mounted over a showerhead assembly includes high temperature (up to 200° C.) rated valves mounted directly thereto, and short, easily purged reactant lines. Integral passageways and metal seals avoid o-rings and attendant dead zones along flow paths. The manifold includes an internal inert gas channel for purging reactant lines within the block inlet manifold